PURPOSE: To detect fine particles with high efficiency, and also, to eliminate a malfunction caused by a noise, etc., by reflecting all scattered light beams by one concave mirror, and leading them to a photoelectric converter.

CONSTITUTION: Two concave mirrors having a spheroidal surface are opposed to each other and arranged on a measuring area by allowing its focuses to coincide. Also, in the other focal positions of the concave mirrors, photoelectric converters 21, 22 are arranged, respectively, therefore, even if a scattered light beam is scattered in any direction, it is reflected by one concave mirror and led to the other photoelectric converters 21, 22. Subsequently, the respective outputs are amplified 23, 24, and next, an adding output by an adder 25 is provided on an analog switch 28. Also, outputs of the amplifiers 23, 24 are multiplied 26 and transmitted to a comparator 27. Accordingly, when a signal exceeding a threshold level of the comparator 27 is applied, the analog switch 28 is closed, and the adding signal is transmitted to an integration circuit 29. Subsequently, an output of the integration circuit 29 is discriminated by a comparator 30, and calculated by a counter 31.