PURPOSE: To obtain an element which operates at a room temp., is simple in construction and permits easy production by forming a laminar material of any among GaS, InS and InSe.

CONSTITUTION: The laminar material is formed of any among GaS, InS and InSe. For example, a substrate 1 is a material which allows the transmission of the light of the wavelength necessary for operation of an optical bistable element and such material is exemplified by glass. A thin film crystal 12 consisting of the material having the large coupling energy of exciton is formed on the substrate 1. The c-axis 22 of the crystal 12 is perpendicular to the plane of the substrate 1 and an optical bistability is obtd. when light is made incident into said direction. More specifically, the optical bistability is obtd. between incident light 3 and exit light 4 when the incident light 3 is made incident approximately perpendicularly to the optical bistable element 21.