To restrict an angle change of a reflection surface of a reflection member that reflects a scanning light beam, an optical scanning device includes a first support portion that supports a reflection mirror at one point at an end in a longitudinal direction of the reflection mirror, a second support portion that supports the reflection mirror at a plurality of points at the other end, a reinforcement portion that reinforces a structure of the second support portion side of a housing in the longitudinal direction of the reflection mirror, compared to a structure of the first support portion side of the housing, one first fixing portion that is used to fix the housing to an image forming apparatus on the first support portion side, and a plurality of second fixing portions that are used to fix the housing to the image forming apparatus on the second support portion side.