A particle counter may include a housing having an inlet, an outlet, and a window therebetween. The inlet and the outlet may be configured such that a fluid can be flowed therethrough. A plurality of light sources may be arranged outside the housing to provide lights of different wavelengths into the housing through the window. Sensors may be provided outside the housing to detect fractions of the lights scattered by a bubble and/or a particle in the fluid. A control part may be configured to monitor intensities of the lights detected by the sensors and to analyze a difference in intensity between the scattered lights, thereby distinguishing the particles from the bubbles in the fluid.