The invention relates to a diffraction grating, a light-receiving element, and an optical head and an optical recording/reproducing apparatus utilizing them and provides a diffraction grating and a light-receiving element capable of detecting a signal of high quality and an optical head and an optical recording/reproducing apparatus utilizing them. A diffraction grating includes a diffracting region which is formed only on a light exit surface and which diffracts and separates first laser light or second laser light emitted by a two-wavelength semiconductor laser to generate a main beam and positive and negative first order sub beams. The depth of concavity formed on the diffracting region is 220 nm. The irregularities are formed such that the pitch length thereof is 22 .mu.m and such that a convexity thereof has a width of 17.6 .mu.m. The ratio of the width of the convexity to the pitch length of the irregularities is 0.8.