A light modulating device 103 includes: a selective diffraction device (10, 10') which generates diffracted light beams of a plurality of orders by diffracting illumination light into one of a plurality of directions, the illumination light being linearly polarized light having a polarization plane oriented in a first polarization direction, and which causes a phase difference between the diffracted light beams of the plurality of orders; and a polarization plane rotating device 14 which rotates the polarization plane of the diffracted light beam of each order so as to be oriented in a direction perpendicular to a direction radiating from an optical axis.
