To provide a particle sensor which discriminates the particle sizes of fine particles, using an inexpensive device, regardless of the concentration of particles.

The particle sensor is equipped with an emitter A and first and second light detectors S1 and S2, producing light detection output signals corresponding to the scattered light of the particles in a measuring area B, based on the LED light emitted from the emitter. The first light detector S1 is arranged behind the measuring area B, positioned in front of the irradiation direction of the LED light from the emitter, and the second light detector S2 is arranged in front of the measuring area B, positioned in front of the irradiation direction of the LED light from the emitter, and the particle sensor is also equipped with a means 1 for calculating the ratio of the light detection output signals by the first and second light detectors and means 2 and 3 for calculating the particle size of the particles from the ratio of the light detection output signals.