PURPOSE: To accurately take synchronization between light emitting and receiving parts by setting the operation completing time of a clock means so as to be set after the recovery of a line voltage.

CONSTITUTION: When a voltage cut off intermittently from the power source supply cutoff means 26 of the light receiving part 2 is supplied to the light emitting part 1, voltage drop detecting means 12, 28 detect the voltages on lines 3, respectively, and when the voltages drop less than a prescribed voltage, a detection signal is outputted to each of first timers 13, 29. The first timers 13, 29 start their operations with the detection signal, and output pulses for first prescribed time. Second timers 14, 30 provided at the output sides of the first timers 13, 29 start their operations synchronizing with the leading or the trailing edgde of the pulse, and when the pulse is outputted for the prescribed time, the light emitting part 1 makes a light emitting device LD emit light for that period, and a smoke detecting operation is performed while the second timer 30 is being operated at the light receiving part 2. In such a way, it is possible to prevent the malfunction of the light emitting and receiving parts occurring even when a source voltage is cut off intermittently.