In a volume phase grating, a plurality of phase gratings for causing a refractive index of a substrate having an incident surface and a launched surface facing each other to be cyclically changed between the incident surface and the launched surface are so formed as to be inclined at a specified angle to the incident surface. The volume phase grating is constructed such that an incident light is caused to be obliquely incident on the incident surface, and an angle of inclination of the phase gratings to the incident surface is set such that the incident light obliquely incident on the incident surface is refracted at the incident surface and perpendicularly incident on the phase gratings.