To provide a diffraction grating which suppresses degradation of diffraction efficiency or improves diffraction efficiency even in the case of presence of a sub-structure different from a main structure constituting projections of the diffraction grating.

A diffraction grating 1 is a binary type and a transmission type diffraction grating the cross-section of which has a repeated structure of projections 2 and recesses on at least one surface of a substrate. In each recess, a sub-structure 3 the cross-section of which is approximately triangular or trapezoidal, is present. A height H of each of the projections 2 and a height h of each of the sub-structures 3 are set so that a ratio of the height h of the sub-structure 3 to the height H of the projection lies within the range of 0.05 to 0.45.