To provide a polarization conversion element with which lowering of light quantity caused by variation of film thickness of an adhesive to adhere transmission plate members to one another is prevented.

The polarization conversion element is equipped with: a first light transparent plate material 2 provided with a polarization separation film 4 formed on a first film forming face 2c, and a warp correction film 11 formed on a second film forming face 2d for correcting a projecting or recessing warp produced due to stress of the polarization separation film 4; a second light transmission plate material 3 bonded alternately with the first light transmission plate material 2 and provided with a reflection film 6 formed on a third film forming face 3c, and a warp correction film 12 formed on a fourth film forming face 3d for correcting a projecting or recessing warp produced due to stress of the reflection film 6 on the film forming face side; and a polarization conversion member 5 to convert polarized light arranged either on an optical path of polarized light having passed through the polarization separation film 4 or on that of polarized light reflected with the reflection film 6.

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