A particle detecting device includes a chamber 30; a first introduction flow path 225 for introducing a particle-containing fluid into the chamber 30; a second introduction flow path 235 for introducing a particle-free fluid into the chamber 30; a light source 10 configured to illuminate fluid in the chamber 30 to detect particles contained in the fluid; a discharge flow path 260 for discharging fluid from the chamber 30; an introduction flow meter 245 configured to measure a flow rate of fluid flowing through the second introduction flow path 235; and a control unit 301 configured to perform control such that a fluid having a total flow rate obtained by adding a predetermined flow rate of fluid flowing through the first introduction flow path 225 to a flow rate of fluid flowing through the second introduction flow path 235, the flow rate being measured by the introduction flow meter 245, flows through the discharge flow path 260.