PURPOSE: To obtain a high optical output level by applying write/readout to an optical bistable element in response to periodic and exchange information so as to set optionally the length of an exchange frame period, allowing to attain miniaturization of the exchange.

CONSTITUTION: A time division optical signal 100 is applied to an optical write switch S120 via a waveguide 110, selected by each time slot and led to any incident terminal of optical bistable elements 130∼160. A bias light is injected to the incident terminal of each element via S132, 142, 152, 162. All elements 130∼ 160 have hysteresis characteristics and when the bias light is applied to the incident terminal, the emitted amount of light has two stable points. The output of each element is transmitted to the waveguide 180 as a time division optical signal 190 via a readout optical switch 170.