An irradiation optical system 12 irradiates a fluid flowing in a flow passage 2a with one light among a plurality of lights obtained by branching light from a light source 1 and forms the detection area. A detection optical system 13 makes scattered light with a different direction from an optical axis of the irradiation optical system enter a beam splitter 17 among the scattered lights from particles contained in the fluid in this detection area. Meanwhile, a beam expander 16 makes another light among the plurality of lights enter the beam splitter 17 as reference light. A detector 4 receives an interference light, by the scattered light and the reference light, obtained by the beam splitter 17 by light receiving elements and generates a detection signal corresponding to the interference light. A counting unit 6 counts the particles based on this detection signal.