Particle counter

A particle counter comprising:

• a light source that emits light;
• a light superimposing unit that spatially superimposes two lights;
• an irradiation optical system that forms a detection area by irradiating a fluid flowing in a flow passage with a first light among a plurality of lights obtained by branching the light from the light source;
• a detection optical system that causes scattered light, among scattered light from a particle included in the fluid in the detection area, that is in a direction different from an optical axis of the irradiation optical system to enter the light superimposing unit;
• a reference optical system that causes a second light among the plurality of lights to enter the light superimposing unit as reference light;
• a detector that receives, using a light receiving element, interference light between the scattered light and the reference light obtained by the light superimposing unit, generates a detection signal corresponding to the interference light, and amplifies the detection signal using an amplifier;
• a counting unit that performs counting of the particle, based on the detection signal in a measurement period for measuring the particle;
• and
• an optical path length variable unit that causes the optical path length of at least one of a first optical path which is an optical path of the first light and the scattered light and a second optical path which is an optical path of the second light to be changed at a predetermined rate,
• wherein the predetermined rate is set based on a flow velocity of the fluid so as to slow a change in a phase difference between the scattered light and the reference light and to make the frequency of the detection signal lower by changing the optical path length.