PURPOSE: To detect fine particles with high efficiency, and also, to eliminate a malfunction caused by a noise, etc., by providing a pair of concave mirrors having a spheroidal surface, whose focuses have been opposed adjacently to each other in a measuring area.

CONSTITUTION: Two concave mirrors having a spheroidal surface are opposed to each other, so that its focuses become adjacent to each other in a measuring area. Also, in the other focal position of each concave mirror, photoelectric converters 21, 22 are arranged, respectively, therefore, even if a scattered light is scattered in any direction, it is reflected by one concave mirror and led to the photoelectric converters 21, 22 of the other. The respective outputs of the photoelectric converters 21, 22 are amplifies 23, 24, and an adding output which has been added 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 transmitted, 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 counted by a counter 31.