When a wavelength having the same value as a reference wavelength .lamda.1
of a first laser light is set to a designed wavelength .lamda., a
diffractive optical element has: an inner circular side irregular
diffraction pattern portion, in which a plurality of irregular portions
in which a height of a convex portion is set to approximately
1.lamda.-fold of the designed wavelength .lamda. with respect to a
concave portion are repeated, being formed in an annular shape in an
inner circular area having a predetermined diameter for the correction of
a spherical aberration generated due to a difference in substrate
thickness between first and second optical recording mediums centering on
a central point through which an optical axis runs while gradually
changing a pitch of the irregular portions in a radial direction toward
the outer circular side; and an outer circular side tiered diffraction
pattern portion which is intended to improve a chromatic aberration with
respect to the first laser light by forming in an annular shape in an
outer circular area outside the inner circular side irregular diffraction
pattern portion a plurality of tiers by setting a height of each tier to
approximately m.lamda.-fold (where m is a natural number which does not
include 0) of the designed wavelength .lamda. or by setting the same
while changing a value of m for each tier.