In a reflection mirror having a concave curved reflection surface, when a surface of the reflection mirror on the side where the reflection surface is formed is a front surface, a surface thereof opposite to the front surface is a rear surface, a direction parallel to an optical axis of the reflection mirror is a first direction, and a direction from the front surface toward the rear surface which is oriented to one side in the first direction is a second direction, the rear surface of the reflection mirror has a flat surface orthogonal to the first direction and a wall surface that rises from at least a part of a region around the flat surface toward the second direction.