A particle counter includes a detector that receives, using a light receiving element, interference light between scattered light and reference light, 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 particle; and an optical path length variable unit that causes the optical path length of at least one of a first optical path and a second optical path 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.