A diffraction display device includes a diffraction optical element that comprises a diffraction display portion and a non-display portion disposed between a pair of substrates, and an illuminating device that emits linearly polarized illuminating light to be incident on a side surface of the diffraction optical element. In the diffraction display device, the illuminating light having entered the diffraction optical element exits the diffraction optical element through a substrate surface at which the diffraction display portion is disposed.