A wavelength-selective polarization conversion element is disclosed, which is capable of reducing the number of optical elements in an optical system and simplifying the configuration thereof. The polarization conversion element comprises a beam splitting film which splits light by transmission and reflection and has a characteristic in which at least its transmittance for light with a first polarization direction changes between a transmittance higher than 50% and a transmittance lower than 50% depending on wavelength regions, and a phase plate which converts the polarization direction of light from the beam splitting film between the first polarization direction and a second polarization direction. The polarization conversion element converts light of two of first, second and third wavelength regions into light with one of the first and second polarization directions and converts light of the remaining wavelength region into light with the other polarization direction.