Disclosed is a diffractive optical element which includes a diffractive grating pattern ion a base plate. The diffractive grating pattern includes a plurality of phase gratings arranged in parallel lines extending along a predetermined direction to cause diffraction of an incident beam. Each of the plurality of phase gratings has an asymmetrical phase pattern. There is a phase gap, &Dgr;P, representing a phase difference (in radians) between an end point and a beginning point of each phase pattern. The phase gap, &Dgr;P, is substantially equal for each of the plurality of phase gratings and satisfies the relationship. 0.7&pgr;