To obtain prism holding structure capable of surely holding a rectangular prism without using an adhesive, and at least mitigating the degradation of optical performance caused by positional deviation between the rectangular prism and a holding member caused by the change of operating environment.

In the prism holding structure for holding the rectangular prism 40 in such a state that it is accurately positioned by the holding member 20, the holding member is equipped with: an insertion port 220; a receiving surface 241 positioned nearly perpendicularly to the opening surface 221 of the insertion port; a supporting part 263 positioned in the vicinity of the insertion port; and a repulsive energizing part 252 exerting repulsive force in the direction of the receiving surface and the supporting part. By repulsively energizing the inclined surface of the rectangular prism where a light entering surface 41, a light emitting surface 42 and the inclined surface 43 cross each other to form a right triangle by the repulsive energizing part, either the light entering surface or the light emitting surface abuts on the receiving surface, and the side edge part of the other abuts on the supporting part, whereby the rectangular prism is held by the holding member.