Optical scanner and image forming apparatus

A disclosed optical scanner includes a light source, a deflector that deflects a light beam emitted by the light source, a scanning imaging element that images and scans the deflected light beam as a light spot on an imaging surface, and an optical box that contains at least the deflector and the scanning imaging element and that is substantially sealed from outside. The deflector is substantially sealed so as to be substantially insulated from the scanning imaging element in the optical box. Among outer walls of the optical box insulating a space inside the optical box from the outside, at least an outer wall positioned above the deflector includes an upside-down U-shaped cross-sectional shape portion.