In an optical information recording/reproducing device which executes information reading or writing on multiple types of optical discs having different data densities by selectively using first through third light beams having first through third wavelengths, respectively, at least one side of an optical element is provided with a step structure including a plurality of concentric refracting surfaces and steps between them. The steps include first steps each giving a first optical path length difference specified by a first optical path difference function, second steps each giving a second optical path length difference specified by a second optical path difference function, and special steps each giving an optical path length difference obtained by the sum or difference of/between the first and second optical path length differences. Annular zone widths between adjacent steps are set at 10 times the first wavelength or more.