A light guide includes a core made of material having translucency and an optical path conversion mirror for reflecting the signal light from the optical element and converting the optical path of the signal light formed on at least the core. The signal light is transmitted through the core by the reflection at the optical path conversion mirror. The optical path conversion mirror surface has an inclination angle α formed with a bottom surface of the core changing in the X-direction in a cross-sectional shape in which the core is cut at a YZ plane, the Y-direction being an optical axis direction of the optical element, the Z-direction being an advancing direction of the signal light of the light guide, and the X-direction being a direction perpendicular to both the Y-direction and the Z-direction.