A particle detecting device includes: a light measuring instrument that measures measured values for intensities of first, second, and third lights of mutually differing wavelengths, produced by particles to be measured; a boundary information storing portion that stores a non-linear discriminating boundary for separating a class of a first classification of particles and a class of a second classification of particles; and a particle classifying portion that classifies the particle being measured into either of the classifications for the first and second classifications of particles, based on measured values for the intensities of the first through third lights and on the discriminating boundary.