An optical multiplexer includes: an incident surface on which incident light beams having different wavelengths are to be incident; a reflection portion configured to reflect the incident light beams; and an emission surface configured to emit reflected light beams reflected by the reflection portion. The incident surface has adjacent condenser lenses corresponding to the respective incident light beams. The reflection portion has adjacent reflection surfaces configured to reflect the respective incident light beams which have been condensed. The adjacent reflection surfaces are respectively disposed so that angle β formed by the respective reflected light beams reflected by the adjacent reflection surfaces is smaller than angle α formed by the respective incident light beams which have been condensed. The emission surface has diffraction grating in which the respective reflected light beams reflected by the adjacent reflection surfaces are to be incident at a same position and diffracted in a same direction.