A driving device includes a stepping motor for causing the light-amount adjusting member to open and close so as to adjust an amount of light passing through an optical system. A driving range where the light-amount adjusting member is actually used for effecting light amount adjustment is set to a range exceeding an electrical angle 360.degree. of the stepping motor. A mechanical abutment portion is provided at a position outside the driving range, and a portion of the light-amount adjusting member is brought into abutment with the mechanical abutment portion when an initial reset of the stepping motor is performed. The light-amount adjusting member is brought to a position a predetermined distance away from the mechanical abutment portion when a light-amount adjusting function of the light-amount adjusting member is stopped.