PURPOSE: To simplify constitution by moving a lens according to the electrical scanning of the line sensor of the face inclined to the direction perpendicular to optical axis.

CONSTITUTION: The subject light having gone through an objective lens 1 and reflected on a half mirror 2 images at the central part of a line sensor 4 of the face inclined to the direction perpendicular to the optical axis. Hence, if the differential component of the detection output of the sensor 4 and the output through a delay circuit 6 by a differential amplifier 7 is processed in a cilipper circuit 8, polarity inversion circuit 9, adder 10 and LPF 11 and the movement of the lens 1 is so controlled that the detection output in the central part of the sensor 4 becomes maximum by way of a phase comparator 12 to which the saw tooth waveform signal synchronized to the line frequency scan-driving the sensor 4 is applied, then focusing is accomplished. Hence, forcusing is accomplished by electrical scanning without machanically moving the image plane, and the constitution of the device becomes simple.