To provide a spatial optical modulation element having a long life free from a deterioration of optical modulation characteristics even if a high power laser beam with an ultra-short pulse is used as a modulation drive light. A spatial optical modulation element 1 wherein disposed between a prism 2 and an optical functional material layer 3 made of an optical functional material with the refractive index changeable by light irradiation, is a low refractive index layer 4 made of a transparent material with a refractive index lower than the refractive index of the dielectric. A spatial optical modulation method characterized in that using such a spatial optical modulation element 1, at the interface between the prism and the low refractive index layer, reflection of a to-be-modulated light 5 entering through the prism, is controlled by a modulation drive light 6.