First and second phase function curves are respectively obtained based on first and second phase functions with a wavelength having the same value as a reference wavelength .lamda.1 of a first laser light being determined as a designed wavelength .lamda., a phase shift element then has an inner circular side tiered phase difference pattern portion formed in annular shapes in an inner circular area on one surface thereof based on the first phase function curve and an outer circular side tiered phase difference pattern portion formed in annular shapes in an outer circular area outside the inner circular side tiered phase difference pattern portion based on the second phase function curve. A tier pitch of tiers of the inner circular side tiered phase difference pattern portion is set to a height corresponding to a phase difference of approximately 1.lamda. and, on the other hand, a tier pitch of tiers of the outer circular side tiered phase difference pattern portion is set to a height corresponding to a phase difference of approximately m.lamda. (where m is a natural number which does not include 0) or a height corresponding to a phase difference of approximately m.lamda. by changing a value of m in accordance with each tier.