A prism optical system provided with: a first prism on which a light ray coming from an objective lens is incident; a second prism on which the light ray coming from this first prism is incident, and from which this incident light ray is emitted; and a plate-like member which is interposed between the aforesaid prisms so as to provide a predetermined air space therebetween and which has a diaphragm opening and a light blocking portion. Further, an uneven portion, which consists of depressions and projections and has a serrate section, for preventing light rays, which have been reflected from the surface of the light blocking portion of this plate-like member, from entering a field stop is provided in the light blocking portion thereof. Thereby, occurrences of ghosts and flares due to stray light are prevented. Moreover, ghosts and flares are reduced.