PURPOSE: To output the presence/absence of an optical signal based upon the presence/ absence of the optical signal by providing with, at least, two bistable semiconductor lasers having a control electrode and a detecting electrode separately and an electrical driving circuit that impresses a signal detected by the detecting electrode on one side on the control electrode on the other side and controls it.

CONSTITUTION: The p-side electrodes of bistable semiconductor lasers 1 and 2 are separated into control electrodes 11 and 21, and detecting electrodes 12 and 22. A laser 1 is started to oscillate when an optical signal R is inputted, and the voltage of the detecting electrode 12 is increased, making a TR4 ON, and a current is on the control electrode 21 in the laser 2. When the optical signal is no more inputted, or when the optical signal R is inputted to the laser 1, a state that the laser 1 is oscillated and the laser 2 is not oscillated is maintained. At such a case, when an optical signal S is inputted to the laser 2, the laser 2 is started to oscillate, making a TR3 ON and the current on the control electrode 11 is reduced fewer than the falling current, terminating the oscillation of the laser 1. As a result, the oscillation of the laser 2 is maintained even in the absence of the optical signal. Thereby, a device functions as a RS flip-flop.