1. Field of the Invention The present invention relates to a control circuit and a control method that control a current provided to a heater configured to heat and melt a solid state device in a melting step of the solid state device by using a heater. 2. Description of the Related Art A method for manufacturing a semiconductor device includes a deposition step of forming an insulating layer on a substrate, a patterning step of forming a trench in the insulating layer, an implantation step of implanting an impurity into the substrate by using the insulating layer as a mask, and the like. A heat treatment is performed as post-processes after each step. Among the post-processes, there is a step to heat and melt a glass substrate in order to form an interlayer dielectric film (SiO2 film) by the chemical vapor deposition method. Such a step is referred to as a melting step. The glass substrate is heated and melted in the heating chamber by supplying a current to the heating element such as a heater and a filament, and the like disposed on the glass substrate. FIG. 13 is a schematic diagram of a solid state device manufacturing apparatus including a melting step. A post-process apparatus 1001 includes a container 1003 for carrying a substrate 1005 in and out, a transfer chamber 1002 for carrying the substrate 1005 between the post-process apparatus 1001 and the container 1003, a load lock chamber 1004 having a function to isolate an external air from the atmosphere, and the like. In the post-process apparatus 1001, while at least one process is performed on the substrate 1005, another substrate 1005 is carried into the load lock chamber 1004 through the load lock chamber 1004, then isolated from the external air, and carried into the post-process apparatus 1001 from the load lock chamber 1004 through the transfer chamber 1002. In the post-process apparatus 1001, there is a melting step of melting a glass substrate with a heating chamber 1006 by heating a heating element 1007 formed on the glass substrate with a filament 1008, and the like. FIG. 14 is a schematic diagram showing a state in which the heating element 1007 is formed on the glass substrate 1005. The filament 1008 is directly connected to the heating element 1007 formed in a frame-like shape. However, since the electrode is not shown, an interlayer insulating layer (SiO2 film) 1009 is formed on a front surface and a back surface of the glass substrate 1005 so as to expose a region to be heated. In the glass substrate 1005, the heating element 1007 and the filament 1008 are embedded in and buried in the interlayer insulating layer 1009. When a current is supplied to the heating element 1007 in this state, the glass substrate 1005 is heated by Joule heat generated by the heating element 1007. FIG. 15 is a cross-sectional view of a state in which the glass substrate 1005 is in a melted state. In a state in which the heating element 1007 is formed, and the like, the glass substrate 1005 is heated so that the glass substrate 1005 is heated to a temperature equal to or higher than a glass transition point, and then the glass substrate 1005 is maintained in a melted state. In this state, while maintaining the glass substrate 1005 at high temperature, a step to remove particles floating in the melting step is performed. When removing the particles floating in the melting step, the semiconductor device manufacturing apparatus is shut down for one or more hours. That is, in order to remove the particles floating in the melting step, the substrate temperature is returned to room temperature (ambient temperature). That is, a current from an electric power source (a direct-current power supply or alternating-current power supply) is stopped. On the other hand, heating of the glass substrate 1005 by the heater (heating element) is continued, and the glass substrate 1005 is maintained in a melted state. The above-described melting step of the glass substrate 1005 is continuously performed until before performing the patterning step or the like. FIG. 16 shows a schematic configuration of a conventional apparatus for controlling the current supplied to the heater in the melting step. The control apparatus 1007 includes a current supply unit 1101, a first switch 1102, and a second switch 1103. The current supply unit 1101 includes a direct-current power supply 1104, an alternating-current power supply 1105, and the like. The current supply unit 1101 includes an alternating-current generator 1106 connected to the alternating-current power supply 1105, and a direct-current power supply 1107 connected to the direct-current power supply 1104. The control apparatus 1007 further includes a control unit 1108 that controls the direct-current power supply 1107 or the alternating-current power supply 1105 of the current supply unit 1101 so that a predetermined current can be supplied to the heater. The first switch 1102 and the second switch 1103 are used to interrupt the current supplied to the heater. The first switch 1102 is configured to interrupt a current from the alternating-current power supply 1105. The second switch 1103 is configured to interrupt a current from the direct-current power supply 1104. The control unit 1108 stops the interruption of the current from the first switch 1102 or the second switch 1103. That is, when starting the operation of the alternating-current generator 1106 of the current supply unit 1101, the control unit 1108 causes the first switch 1102 to start switching on and off so that the direct-current power supply 1104 is connected to the alternating-current power supply 1105, and causes the second switch 1103 to start switching on and off so that the alternating-current generator 1106 is not operated. When ending the operation of the alternating-current generator 1106 of the current supply unit 1101, the control unit 1108 causes the first switch 1102 to start switching off so that the current is interrupted from the alternating-current power supply 1105, and causes the second switch 1103 to start switching off so that the alternating-current generator 1106 is not operated. In a state in which the current is not supplied to the heater, the glass substrate 1005 is maintained in a melted state. When maintaining the glass substrate 1005 at high temperature, it is difficult to maintain a predetermined temperature using a method in which a heating element and the like such as the above-described heating element 1007, and the like are used. That is, even if a predetermined voltage is applied to the heating element 1007, the heating element 1007 cannot maintain a stable voltage. Accordingly, a current with a prescribed temperature is constantly supplied to the heating element 1007. Meanwhile, when interrupting the current from the second switch 1103, if the current is not interrupted sufficiently, a heating time of the heater (a heater power loss) increases. Accordingly, there is a problem in that the heating time required to melt the glass substrate 1005 may not be obtained within a limited time. In this regard, the above-described control apparatus 1007, and the like that interrupt the current from the second switch 1103 is not disclosed. That is, the control apparatus 1007 is not used so that the current can be efficiently supplied to the heating element 1007. Accordingly, when melting the glass substrate 1005, the heating element 1007 may not be maintained at the prescribed temperature (the target temperature), and accordingly, the heating element 1007 may be thermally damaged.