An optical device comprising a first optical path having a first optical axis, a second optical path having a second optical axis not parallel to the first optical axis, and a mirror arranged to move across a bisector of an angle between the first and second optical axes. The mirror has a surface including a reflecting portion for reflecting light from the first path toward the second path. The reflecting portion has an edge including a linear portion placed on a plane substantially perpendicular to the bisector. The linear portion is inclined relative to a normal to a plane including the first and second optical axes.