Disclosed herein is a diffractive waveguide-spatial optical modulator. The diffractive waveguide-spatial optical modulator includes a substrate member, a plurality of reflecting members, and an actuation means. The reflecting members are each formed in a plate shape, are arranged on the substrate member at regular intervals to form a grating array, are configured to form openings therebetween, and are provided with reflecting surfaces formed on the opposite vertical surfaces of the reflecting members and the bottoms of the openings. The actuation means varies the interval between the reflecting members by actuating the reflecting members. The openings between the reflecting members act as wave guides when light is incident on the open sides thereof, and shift the phase of the incident light and then reflect the incident light, so that the reflected light can form diffracted light, when the interval is varied by the reflecting members.