A transmission grating that provides low polarization dependent loss over a wide wave range and provides high diffraction efficiency even with a small groove pitch and high resolving power and dispersion. In a transmission grating 10, multiple parallel ridges 22 that are transparent for the wave range to be used are disposed on one side of a substrate 20 that is transparent for the wave range to be used. Parallel grooves 24 are formed at a fixed pitch a between these ridges. Light is applied from the surface of the transmission grating on which the grooves are formed and diffracted light is extracted from the substrate surface on which grooves are not formed. The groove pitch a is set to a range of 0.51 λc-1.48 λc, where λc is the central wavelength.