A particle counting method that can count the number of the particles precisely. The method discriminates a wave pattern of the scattered light from a normal particle (subject of the counting) and a wave pattern of the light scattered by the agitation such as a floating particle, a radiation or changes in the intensity of the light. In one embodiment, a method for counting particles is disclosed which irradiates a light to a sample gas, detects a scattered light from a particle included in the sample gas by a photoelectric conversion device, counts the number of the particles of every particle size division by the output voltage wave pattern of the photoelectric conversion device, calculate a time difference (Ta−T1) from a point (T1) being a peak of output voltage wave pattern and a point (Ta) being a falling detection threshold (A), when the time difference (Ta−T1) is beyond counting cancellation time (B), the output voltage wave pattern is not counted as a particle.