PURPOSE: To stabilize and improve the optical output of a light emitting element for an optical modulating circuit containing a light emission output stabilizing function, by transmitting a part of the modulated signal through a delay circuit before supplying it to a difference signal detecting circuit.

CONSTITUTION: A delay circuit 10 containing an optical connecting circuit consisting of a semiconductor laser 12 and a photodetecting element 13 is set on a signal route before a part of the modulated signal supplied from a terminal 1 is supplied to a difference signal detecting circuit 7. Therefore the output signal of the circuit 7 produces no undesired error signal due to a difference of delay time by applying a signal waveform showing the optical output of a semiconductor laser 4. Thus the optical output of the laser 4 can be extremely stabilized and improved.