A diffractive lens according to the present invention has the function of focusing light. The diffractive lens has a side on which a diffraction grating is arranged on either an aspheric surface or a spherical surface in its effective area. The diffraction grating has n0 phase steps, which are arranged concentrically around the optical axis of the diffractive lens. And the radius rn of the circle formed by an nth one (where n is an integer that falls within the range of 0 through n0) of the phase steps as counted from the optical axis of the diffractive lens satisfies rn=√{square root over (a{(n+c+dn)−b(n+c+dn)m})}{square root over (a{(n+c+dn)−b(n+c+dn)m})} where a, b, c and m are constants that satisfy a>0, 0≦c 1, and 0.05 ⁢ b 0