PURPOSE: To reduce the size and weight of a light scatter type particulate sensor by using a light emitting diode as the light source of the light scatter type particulate sensor and arranging the light source in a detection cell to which the scattered light converges.

CONSTITUTION: Sample air S Which contains particulates ins sent in the detection cell C through an air feed nozzle 6 and a suction nozzle 7 together with sheath air. Parallel light L from the light emitting diode in the light source 3 is irradiated and scattered light from particulates D is incident on a light receiving element 8 through a spherical reflector 4 and a condenser lens 2. A through hole 4a is formed at the center part of the spherical reflector 4 and part of the light L passed through the through hole 4a is photodetected by a photodiode 5 in the front, thereby controlling the light L to constant intensity. Consequently, a pulse signal from the light receiving element 8 is counted through a comparator, etc., to accurately detect the number of particulates.