Light-amount detecting device, light source device, optical scanning unit and image forming apparatus

A light-amount detecting device includes: a light source which emits a light beam; a branching optical element which divides the light beam emitted from the light source into a first light beam traveling in a predetermined direction and a second light beam traveling in a direction different to the predetermined direction; a light-condensing element which condenses the second light beam; a light-receiving element having a light-receiving surface which receives the second light beam condensed by the light-condensing element; and a detector which detects a light-amount of the second light beam received by the light-receiving element, and at least one of a direction of reflected light of the second light beam reflected from the light-receiving surface of the light-receiving element and spread of the reflected light of the second light beam reflected from the light-receiving surface of the light-receiving element is adjusted to control a light-amount of the reflected light of the second light beam returning to the light source.