The present invention relates generally to a fluid-filled flexible bag and a fluid container using such a bag, the fluid-filled flexible bag being made of polyethylene-based resin, polyester-based resin, nylon-based resin or polypropylene-based resin and used for containing a fluid such as food, a liquid chemical, blood, plasma, or the like. The fluid container using the fluid-filled flexible bag of this type is widely used because it can be easily produced by blow molding and it has superior resistance to chemicals. In a conventional fluid container, however, the fluid-filled flexible bag is apt to be broken at the time of inflation because the fluid-filled flexible bag is made of polyethylene-based resin, polyester-based resin, nylon-based resin or polypropylene-based resin having a relatively low strength. Accordingly, the bag should have a very weak and thin wall in order that the fluid may be contained by the bag. If the bag has a weak wall, the container cannot hold the fluid at a high pressure. In this connection, an additional means for holding the bag at a high pressure is required. In the light of the foregoing, an object of the present invention is to provide a fluid-filled flexible bag having an excellent resistance to chemicals and a fluid container using such a bag. Another object of the present invention is to provide a fluid-filled flexible bag which can be easily inflated and deflated at low costs and a fluid container using such a bag. A further object of the present invention is to provide a fluid-filled flexible bag which has a thick wall even if a part of the bag is folded and used for containing fluid therein, and a fluid container using such a bag. A still further object of the present invention is to provide a fluid-filled flexible bag which can be easily sealed with sealing means of a fluid container using such a bag. According to the present invention, there is provided a fluid-filled flexible bag including an inner layer, a thick outer layer formed on the inner layer, and a part of an outer layer formed on the inner layer, wherein at least a part of the outer layer is thicker than a thickness of the inner layer. According to the present invention, there is further provided a fluid-filled flexible bag including an inner layer, a thick outer layer formed on the inner layer, a partition wall formed between the inner layer and the outer layer, and a partition-wall-forming portion of the outer layer formed on the inner layer, wherein at least a part of the outer layer is thicker than a thickness of the inner layer. According to the present invention, there is still further provided a fluid container including a container main body, a fluid-filled flexible bag having an inner layer, a thick outer layer formed on the inner layer, and a part of an outer layer formed on the inner layer, the flexible bag being installed in the container main body, and wherein at least a part of the outer layer is thicker than a thickness of the inner layer. According to the present invention, there is still further provided a fluid container including a container main body, a fluid-filled flexible bag having an inner layer, a thick outer layer formed on the inner layer, a partition wall formed between the inner layer and the outer layer, and a partition-wall-forming portion of the outer layer formed on the inner layer, the flexible bag being installed in the container main body, and wherein at least a part of the outer layer is thicker than a thickness of the inner layer. According to the present invention, there is still further provided a fluid container including a container main body, a fluid-filled flexible bag having an inner layer, a thick outer layer formed on the inner layer, a partition wall formed between the inner layer and the outer layer, a partition-wall-forming portion of the outer layer formed on the inner layer, the flexible bag being installed in the container main body, and wherein the partition wall is formed in such a manner that the outer layer is thicker than the inner layer. According to the present invention, there is still further provided a fluid container including a container main body, a fluid-filled flexible bag having an inner layer, a thick outer layer formed on the inner layer, and a partition wall formed between the inner layer and the outer layer, the flexible bag being installed in the container main body, wherein at least a part of the outer layer is thicker than a thickness of the inner layer, and wherein the partition wall is formed in such a manner that the outer layer is thicker than the inner layer. The fluid-filled flexible bag and the fluid container using such a bag according to the present invention has excellent resistance to chemicals because the bag is made of polyethylene-based resin, polyester-based resin, nylon-based resin or polypropylene-based resin. Moreover, in spite of the presence of the thick outer layer, the bag can have a thick wall, as it were, even if the bag is folded and used for containing fluid therein. As a result, the fluid-filled flexible bag can be easily inflated and deflated at low costs. The fluid-filled flexible bag and the fluid container using such a bag according to the present invention has a thick outer layer. Therefore, the partition wall for forming the fluid-filled flexible bag does not become so thin. Thus, the fluid container has a high fluid holding pressure. The fluid-filled flexible bag and the fluid container using such a bag according to the present invention has an outer layer which is thicker than the inner layer in which the flexible bag is installed. Therefore, the outer layer does not interfere with the flexible bag which is installed in the container main body and is formed on the inner layer. Thus, the bag can be sealed with sealing means of the container main body. The fluid-filled flexible bag and the fluid container using such a bag according to the present invention has an outer layer having a portion formed on the inner layer. In this way, the bag is formed by folding a part of the outer layer formed on the inner layer so that the portion of the outer layer is formed on the inner layer. The bag according to the present invention can have an excellent resistance to chemicals and is easily inflated and deflated because the outer layer is formed on the inner layer. The fluid-filled flexible bag and the fluid container using such a bag according to the present invention have an outer layer which is thicker than the inner layer. In this way, the outer layer can be prevented from being so thin as not to be suitable for inflation. Thus, the bag can be inflated and deflated at low costs. The present invention includes a partition wall in the bag. In this case, the bag has a partition wall which is formed by folding a part of the outer layer of the bag which is formed on the inner layer, and therefore has an excellent resistance to chemicals. Moreover, the present invention includes a partition wall in the bag, the bag having a partition wall which is formed in such a manner that the outer layer is thicker than the inner layer. In this case, the partition wall of the bag can be easily formed. Therefore, the bag can have a high pressure holding ability. Moreover, the present invention includes a partition wall in the bag, the bag having a partition wall which is formed in such a manner that the outer layer is thicker than the inner layer. In this case, the bag can be easily sealed with sealing means of the fluid container in which the bag is installed. In the present invention, an outer layer of the bag which is installed in the container main body may be the same as an outer layer of the flexible bag. However, the outer layer of the bag may be thicker than the inner layer of the flexible bag. In the present invention, it is possible to adopt the following configurations: the flexible bag includes a sealing film formed on the inner layer, and the sealing film forms the partition wall together with the inner layer; and the sealing film is provided with a film-constituting layer for sealing. In the present invention, a partition wall may be formed in such a manner that at least a part of a side wall of the container main body is thicker than a side wall of the flexible bag.