A reticle unit equipped with an optical apparatus such as a riflescope includes: red and green light sources radiating red light and green light; a disc-like shaped reticle substrate that has a diffraction grating formed at substantially the center of a surface having substantially circular shape; and red and green mirror members converge light radiated from the red and green light sources, make incidence on the reticle substrate from corresponding side surface portions of the reticle substrate to illuminate the diffraction grating so as to emit first order diffracted light reflected and diffracted by the diffraction grating along a normal direction of the diffraction grating.