PURPOSE: To prevent the deviation of a step motor from an origin by forming clearance of one or less step angle in terms of the step angle of the step motor, in the mutual engaging point of a main driving member with a lens driving member and the mutual engaging point of the main driving member with a cam member, in the original position.

CONSTITUTION: In the original position of the main driving device 1, the clearance of one or less step angle in terms of the step angle of the step motor 4 is formed in the mutual engaging point of the main driving member 1 with the lens driving member 8, so that, reverse energizing force applied to the lens driving means 8 is not transmitted to the step motor 4. In the same way, since the clearance of one or less step angle i terms of the step angle of the step motor 4 is formed in the mutual engaging point of the main driving member 1 with the cam member 16, normal energizing force applied to the cam member 16 is not transmitted to the step motor 4. That is to say, the reverse energizing force applied to the lens driving means 8 and the normal energizing force applied to the cam member 16 are both set to be larger than the stopping and holding force of thee step motor 4 in a non-electric condition and smaller than the revolving force of the step motor 4 in an electric condition. Therefore, deviation from the origin does not arise in the step motor 4.