PURPOSE: To generate an alarm sound when particulates reach a set value by letting the particulates contained in the sample air scatter a laser beam.

CONSTITUTION: An exhaust pipe 4 is connected to the lower end of an exhaust nozzle 3b. A suction pump 5 sucks the outside air into a casing 1 via a suction pipe 2 and a suction nozzle 3a. A flow adjusting apparatus 6 adjusts the flow rate of the air. When the sample air passes an opening G, an irradiating unit 7 irradiates laser light, which is scattered by particulates in a sample and a light trap 19 absorbs the laser light which is not scattered. Two aspheric lenses 17 and 18 collect the scattered light to feed it to a photodetector 21, which converts the scattered light to electric signals. The signal processing system of a printed circuit board 22 amplifies the electric signals, analyzes a waveform, counts the signals and, when a given value set in advance is reached, the signal processing system can operate an alarm.