PURPOSE: To discriminate immediately whether the driving control of an optical system is normal or not, by detecting whether the time of movement of the optical system is between the first preliminarily determined set time and the second set time longer than the first set time or not.

CONSTITUTION: A position which an optical system 1 reaches at a time point from the start of forward movement is defined as a reference position and is indicated with F, and a position which the optical system is moved to in the direction of forward movement from the reference position, for example, a backward movement start position is indicated with S0, and the first preliminarily determined set time is indicated with t1 (t1>t0). When the time of movement of the optical system at a time point with the position F as the reference is denoted as (t), it is discriminated whether t>t0 is true or not; and if it is not true, it is discriminated whether s≥s0 is true or not. The unavailability of a speed controlling function of the optical system is discriminated immediately if t>t0 and s≥s0 are true, and a motor is stopped by this detection to hold the safety.