In a cap for a semiconductor device in which a light transmissive window is fixed to a cap body provided with a light transmissive opening using low-melting glass as a fixing material so that the light transmissive window covers the light transmissive opening, the low-melting glass is leadless vanadate-series low-melting glass, and the light transmissive window is fixed to the cap body through an eutectic alloy layer formed by an eutectic reaction of vanadium contained in the low-melting glass and metal applied on the surface of the cap body.