A diffraction grating with periodically arranged protrusions and grooves at a relatively narrow pitch and improved diffraction efficiency is disclosed. The protrusions of the grating are made of a material whose index of refraction is greater than that of the grooves, and the ratio of the width D of the protrusion to the pitch .LAMBDA. of the protrusion is set equal to or less than 0.4 (D/.LAMBDA..ltoreq.0.4).