A particle monitor system capable of accurately detecting the number of or the size of particles flowing through an exhaust pipe. In a bypass line through which a chamber is communicated with a dry pump, there is disposed the particle monitor system that includes a laser oscillator for irradiating laser light, a photo multiplier tube having a focal point thereof located at a location where the center axis of the bypass line crosses the laser light, and a particle converging member formed by a circular disk-like member and formed with a through hole facing the focal point FP. A gap is defined between the bypass line and an outer periphery of the particle converging member.