To provide a wavelength-selective diffraction element which secures diffraction efficiency suited to different wavelengths and prevents or suppresses occurrence of unnecessary diffracted light, to provide an optical pickup provided with the wavelength-selective diffraction element, and to provide an optical information processor and an optical information processing method using the optical pickup.

The wavelength-selective diffraction element has a portion 41 formed of first material and a portion 42 formed of second material which are arranged alternately along a substrate 43 transmitting the light L2 of a wavelength 1 and the light L1 of a wavelength 2. The refractive index of the first material is different from that of the second material. The portion 41 is provided with a sub wavelength structure having a pitch smaller than the wavelength 1 and the wavelength 2. The effective refractive index of the sub wavelength structure and the refractive index of the second material are equal to each other with respect to the light of the wavelength 1 but different from each other with respect to the light of the wavelength 2. The optical pickup is provided with such a wavelength-selective diffraction element. The optical information processor and optical information processing method use this optical pickup.