A transmission grating includes: first light-transmissive regions having a refractive index of n1; and second light-transmissive regions having a refractive index of n2 that is smaller than n1. Light-reflecting interfaces on which light transmitted through the first light-transmissive regions is incident are in parallel with one another and are inclined such that a line normal to each of light-reflecting interfaces is at an inclination angle θ with respect to the flat light-incident surface and to the flat light-emitting surface, wherein 0°