An enclosure of a dichroic mirror system that synthesizes a plurality of monochromatic laser beams emitted from a laser light source to output synthesized light is a structure having a dichroic mirror system lower enclosure and a dichroic mirror system upper enclosure joined to each other, and includes a light-leakage protecting structure that is formed by a convex portion formed at an end of the dichroic mirror system lower enclosure and a concave portion formed at an end of the dichroic mirror system upper enclosure engaged with each other at a joint therebetween.