A light modulator has two unit devices (1,2) each using surface plasmon waves generated at the interface between thin metal films (13,14) formed respectively on prisms (11,12) and an electro-optical material (15), and a mirror (3). Both the transmitted light due to absorption and re-radiation, and the reflected light arising from the unit devices (1,2) appear in the outgoing light, the incident light on the next unit device, or the incident light on the mirror (3). Consequently, all light beams can be utilized as the final outgoing light beams with no loss of light. Further, the colour of light can be spatially divided, and still further, it can also be temporally divided by changing the wavelength by means of a voltage. As a result, the original light can be divided both temporally and spatially with almost no loss by combining two unit devices so configured as to re-radiate the absorbed light by the surface plasmon waves using surface plasmon and a mirror, and thus utilize both the reflected light and the transmitted light.