Abstract: An optical transmitter receiver is provided in which the power of light to be transmitted therefrom is set to a proper value by controlling the relative timing of changing the power of light to be transmitted therefrom. An optical transmitter receiver consists of an optical transmitter, an optical receiver, a signal level detector, and a signal level controller. The optical transmitter transmits a light signal to a mate optical transmitter receiver. The optical receiver receives a light signal from the mate optical transmitter receiver. The signal level detector detects the level of the light signal received by the optical receiver. The signal level controller changes the power of the light signal to be transmitted by the optical transmitter from one value to another according to the signal level detected by the signal level detector.