A light modulator which modulates light which propagates in a propagation direction, comprising: a first electro-optic crystal substrate that has a first periodic polarization reversal structure and lets the light through the first periodic polarization reversal structure in which first polarization pairs, in each of which the directions of polarization developing in response to electric fields are opposite to each other, are arranged in a first period along a first arrangement direction which is orthogonal to the propagation direction; a second electro-optic crystal substrate that has a second periodic polarization reversal structure and lets the outgoing light from the first electro-optic crystal substrate through the second periodic polarization reversal structure in which second polarization pairs, in each of which the directions of polarization developing in response to electric fields are opposite to each other, are arranged in a second period along a second arrangement direction which is orthogonal to the propagation direction; and a third electro-optic crystal substrate that has a third periodic polarization reversal structure and lets the light through the third periodic polarization reversal structure in which third polarization pairs, in each of which the directions of polarization developing in response to electric fields are opposite to each other, are arranged in a third period along a third arrangement direction which is orthogonal to the propagation direction, a driver that applies a voltage upon the first electro-optic crystal substrate, the second electro-optic crystal substrate and the third electro-optic crystal substrate to create diffraction gratings inside the first periodic polarization reversal structure, the second periodic polarization reversal structure and the third periodic polarization reversal structure, thereby modulating the light, and creating a Raman-Nath diffraction, characterized in that the three electro-optic crystal substrates are disposed in series along the propagation direction of light (a direction Z) on a hold plate, which is orthogonal to the arrangement directions of the periodic polarization reversal structures, and wherein the first period and the third period are the same, the second period is twice the first period. a first electro-optic crystal substrate that has a first periodic polarization reversal structure and lets the light through the first periodic polarization reversal structure in which first polarization pairs, in each of which the directions of polarization developing in response to electric fields are opposite to each other, are arranged in a first period along a first arrangement direction which is orthogonal to the propagation direction; a second electro-optic crystal substrate that has a second periodic polarization reversal structure and lets the outgoing light from the first electro-optic crystal substrate through the second periodic polarization reversal structure in which second polarization pairs, in each of which the directions of polarization developing in response to electric fields are opposite to each other, are arranged in a second period along a second arrangement direction which is orthogonal to the propagation direction; and a third electro-optic crystal substrate that has a third periodic polarization reversal structure and lets the light through the third periodic polarization reversal structure in which third polarization pairs, in each of which the directions of polarization developing in response to electric fields are opposite to each other, are arranged in a third period along a third arrangement direction which is orthogonal to the propagation direction, a driver that applies a voltage upon the first electro-optic crystal substrate, the second electro-optic crystal substrate and the third electro-optic crystal substrate to create diffraction gratings inside the first periodic polarization reversal structure, the second periodic polarization reversal structure and the third periodic polarization reversal structure, thereby modulating the light, and creating a Raman-Nath diffraction, characterized in that the three electro-optic crystal substrates are disposed in series along the propagation direction of light (a direction Z) on a hold plate, which is orthogonal to the arrangement directions of the periodic polarization reversal structures, and wherein the first period and the third period are the same, the second period is twice the first period.