The present invention relates to a diffractive optical element which has chromatic
aberration capable of recording and reproducing without any problem even in the
case of instantaneous wavelength change in a laser beam that cannot be followed
by an actuator, and can control an adverse effect caused on a spot diameter of
a converged spot and on a signal read by a sensor. The diffractive optical element
includes a diffractive structure, and diffraction efficiency of mth order
diffracted beam satisfies prescribed conditions, and a distance between a paraxial
converging position of the diffracted beam having a higher diffraction efficiency
among (m+1)th order diffracted beam and (m-1)th order diffracted
beam and a paraxial converging position of the mth diffracted beam satisfies
prescribed conditions, when m represents a diffraction order number of the diffracted
beam used for the first optical information recording medium.