An exposing device includes a light source that emits scanning light, a polygonal rotating mirror that scans and deflects the scanning light in a specific scanning direction, a reflection mirror that reflects the scanning light that has been scanned and deflected at plural reflection points aligned in a specific direction for the scanning light to go incident on the image carrier, and a unit main body in which the polygonal rotating mirror and the reflection mirror are disposed at a specific interval. The unit main body has two attachment fixing portions on almost a same line linking the reflection points and fixed to the apparatus main body. An attachment support is supported on the apparatus main body and allows free expansion of the unit main body in a plane direction including the scanning direction of the polygonal rotating mirror on a side where the polygonal rotating mirror is installed.