Disclosed herein is a variable grating diffractive light modulator, which actuates lower reflecting plates of a lower reflecting array by a piezoelectric actuating method, and includes an upper reflecting array that is spaced apart from the lower reflecting array and attached to a lower surface of a substrate that is made of a light transmissive material and covers the lower reflecting array, so that the light modulator reflects incident light transmitted through the substrate and thereby produces diffractive light.