An optical scanning device, comprising: a light emitting element for simultaneously emitting light beams in a first direction and a second direction; an optical deflector (10, 49), including a mirror unit configured to reflect the light beam emitted in the first direction, configured to draw a scanning line by the light beam by driving the mirror unit; a first light receiving element configured to receive the light beam reflected by the mirror unit; a reference signal generator configured to generate a reference signal, which acts as a reference to a timing at which the drawing of the scanning line is started, based on a signal output from the first light receiving element; a second light receiving element configured to receive the light beam emitted in the second direction; a light quantity controller configured to measure a signal output from the second light receiving element and automatically controlling the light quantity of the light beam emitted from the light emitting element to a target value during an automatic control period configured to automatically control the light quantity of the light beam emitted from the light emitting element; and a drive controller configured to control the drive of the mirror unit, wherein the light quantity controller starts the automatic control at a timing at which the light beam reflected by the mirror unit passes through the position of the second light receiving element and is received by the first light receiving element and completes the automatic control before the light beam reflected by the mirror unit reaches the position of the second light receiving element next, characterized in that the light quantity controller performs the automatic control after causing the drive controller to perform a control to reduce a rotation speed of the mirror unit if the automatic control is not completed before the light beam reflected by the mirror unit reaches the position of the second light receiving element next. a light emitting element for simultaneously emitting light beams in a first direction and a second direction; an optical deflector (10, 49), including a mirror unit configured to reflect the light beam emitted in the first direction, configured to draw a scanning line by the light beam by driving the mirror unit; a first light receiving element configured to receive the light beam reflected by the mirror unit; a reference signal generator configured to generate a reference signal, which acts as a reference to a timing at which the drawing of the scanning line is started, based on a signal output from the first light receiving element; a second light receiving element configured to receive the light beam emitted in the second direction; a light quantity controller configured to measure a signal output from the second light receiving element and automatically controlling the light quantity of the light beam emitted from the light emitting element to a target value during an automatic control period configured to automatically control the light quantity of the light beam emitted from the light emitting element; and a drive controller configured to control the drive of the mirror unit, wherein the light quantity controller starts the automatic control at a timing at which the light beam reflected by the mirror unit passes through the position of the second light receiving element and is received by the first light receiving element and completes the automatic control before the light beam reflected by the mirror unit reaches the position of the second light receiving element next, characterized in that the light quantity controller performs the automatic control after causing the drive controller to perform a control to reduce a rotation speed of the mirror unit if the automatic control is not completed before the light beam reflected by the mirror unit reaches the position of the second light receiving element next.