Building and accessing an REL database

A device for automatically focusing an objective lens includes detecting means capable of detecting the positional relation of the objective lens with respect to an in-focus position at which an image of an object is formed on a predetermined focal plane, means including an electric motor and moving the objective lens toward the in-focus position by the motor, and means for controlling the driving of the motor in response to the detecting means. The control means includes a power supply circuit means for connecting the motor to a power source, a braking circuit means for short-circuiting the two terminals of the motor, and means for alternately operating the power supply circuit means and the braking circuit means to thereby decelerate the revolution of the motor when the objective lens is within a predetermined range near the in-focus position.