The present invention relates to a portable, compact, lightweight and inexpensive magnetic drive device which is capable of providing a large torque for rotating an armature for a large displacement and at a high speed. Japanese Laid-Open Patent Application No. S58-203438 discloses a prior art electric motor capable of providing a large torque. The electric motor has an annular armature secured to a shaft and spaced apart from the shaft by a suitable distance. The armature is coupled to the shaft by gears. A drive current is applied to the armature for rotation of the armature. The armature and the shaft are coupled to each other by the gears, which in turn results in the shaft being rotated. This causes a magnet retained by the armature to move in a radial direction thereof to provide an effective magnetic flux. Therefore, when the drive current is applied to the armature, the magnetic flux increases to provide a large torque and the speed of rotation of the armature is increased to a maximum extent. The prior art magnetic drive device suffers from a drawback that the magnetic flux provided by the magnet is not constant. As the shaft is rotated, the armature slides with respect to a magnet retaining member and slides with respect to the shaft. This results in the armature becoming unstable. Thus, the shaft is not stably supported. As a result, the magnetic drive device can not be rotated at a sufficiently high rotational speed. In addition, this magnetic drive device is not suitable for use as an electric motor of a power window system for an automobile. In other words, when this device is used as an electric motor of a power window system, the armature is rotated at a speed which is almost constant in the entire range of rotational angles of the armature, and the rotational speed changes by only very little amount. The rotational speed of the armature increases in a range of rotational angles of the armature in which the armature is rotating to its high rotational speed side, while the rotational speed decreases in a range of rotational angles in which the armature is rotating to its low rotational speed side. Thus, the prior art electric motor of a power window system does not permit the armature to be rotated at the maximum speed. Therefore, the prior art electric motor of a power window system is not suitable for use as a direct-current motor for driving a power window system which opens or closes a window glass of a power window system at a high speed, for example, at a speed of 1000 rpm or more, and/or drives a power window system at a high torque. It is to be noted that the magnitude of torque produced by the above-mentioned prior art magnetic drive device depends on the magnitude of current applied to the armature. If the armature is moved from a position in which the drive current is zero to a position in which the drive current has a predetermined magnitude, the armature begins to rotate when the drive current reaches its predetermined magnitude. The amount of current which flows through the armature while the armature is rotating is determined by the magnitude of the drive current and the amount of time that the drive current flows, that is, by the drive frequency. As the drive frequency is increased, the amount of current flowing through the armature is increased. As the amount of current increases, the amount of torque produced by the magnetic drive device also increases. Thus, if the drive frequency is increased, the magnetic drive device can be driven at a higher rotational speed, and therefore the prior art electric motor of a power window system permits a window glass to be opened or closed at a higher speed. On the other hand, the amount of current flowing through the armature is determined by the magnitude of the drive current and the amount of time that the drive current flows. When the drive current is applied at a frequency which is not too high and is low, the armature can be moved at a high speed. However, the armature cannot be moved at a sufficiently high speed if the drive current is applied at a frequency which is not too low and is high. In view of the above, it is an object of the present invention to provide a magnetic drive device which permits the drive speed to be changed between high and low values and which provides a high torque even if the amount of drive current applied thereto is limited. According to the present invention, there is provided a magnetic drive device comprising an annular armature mounted to a shaft and spaced apart from the shaft. A magnet is secured to the armature and has a field magnetic flux density which decreases from a central portion of the magnet toward peripheral portions of the magnet in a direction along a direction of a rotating axis of the armature. The magnet is rotated by a driving force applied to the armature. A magnetic path is provided for allowing the field magnetic flux density of the magnet to be changed between a first value and a second value, the second value being higher than the first value. According to the present invention, the magnetic drive device is used in combination with a driving device including an electric motor. The electric motor has a field magnet having a magnetically attractive force which decreases from a central portion thereof toward peripheral portions thereof in a direction along the direction of a rotating axis of the electric motor. The electric motor also has a rotatable armature. The rotatable armature is positioned to be magnetically attracted to the field magnet and is rotated by the field magnet which provides the magnetically attractive force, thereby producing a driving force in accordance with the magnitude of the magnetically attractive force. The magnetic drive device further comprises a magnetic flux changeover member which extends radially outwardly of the armature and contacts an inner circumferential surface of the armature, a drive mechanism for rotating the armature about the shaft, a driving force transmission mechanism for converting the driving force applied to the armature into a rotational speed of the armature, and a speed control mechanism for changing the rotational speed of the armature between a first rotational speed and a second rotational speed which are equal to each other. The magnetic flux changeover member is a first magnetic flux changeover member. The first magnetic flux changeover member is located at a peripheral position of the armature and has a first portion and a second portion which extend radially outwardly of the armature, and which are spaced apart from each other by a first gap. The first magnetic flux changeover member causes the field magnetic flux density of the magnet to be changed when the first portion contacts the armature. The first magnetic