PURPOSE: To improve electromagnetic noise resistance and process logical operations in parallel by constituting a purely optical logical element based upon the optical bistability of a semiconductor material.

CONSTITUTION: A semi-insulating substrate 1 is chemically etched off to form a window 3 for exposing a multiple quantum well MQW where the MQW is grown on the substrate 1 to perform logical processing, i.e. where squares with a 100μ side length are arranged in parallel at intervals of 100μm. For example, three optical fibers 4 for transmitting an input signal are arranged at one side of each window 3 and fixed so that input light beams from the optical fibers meet one another in the center of the window 3, and one fiber 5 for transmitting output light is arranged and fixed at the opposite side. Then, an input signal from a semiconductor laser as a light source is supplied nd the level of the input signal is set properly to the threshold value of the optical bistability of the MQW to realize logical operations.