an image pickup optical system comprising a stationary lens system, a movable lens system and an image pickup device for converting an optical image into an electric image signal, at a predetermined cycle and outputting the electric signal, wherein the stationary lens system, the movable lens system and the image pickup device are arranged in this order from an object;

a focusing device;

an optical flow calculation device for calculating optical flows in accordance with said image signals in a first image plane at a first time and in a second image plane at a second time, said second time being different from said first time, and for calculating an average of magnitudes and directions of said optical flows; and

a judgment device for determining whether or not the focusing device is to be driven in accordance with said calculated average of magnitudes and directions of said optical flows,

wherein said optical flow calculation device is further configured to total the magnitude of said optical flows after said calculations, and to weigh the magnitude of said optical flows by the flowing relationship:

L×(Scnt−Snum)/Scnt

wherein

L designates the magnitude of said optical flows:

Scnt designates the total number of drive steps necessary for moving the movable lens system from a furthest position to a closest position; and

Snum designates the number of drive steps necessary for moving the movable lens system from the furthest position to a present position.