To provide a monodisperse air suspended particle classifier which permits the production of monodisperse standard particles having a particle size of 0.1 to 1.0 μm.

Non-charged particles distributed so as to contain particles having a desired particle size are guided to a chamber 11. A photo ionizer 16 capable of adjusting soft X-ray output is attached to the chamber 11 and charges the particles in the chamber 11. Therein, the level of the soft X-ray output is adjusted so as to become single charge by a controller 17. Subsequently, the charged particles are guided to a differential electric mobility analyzer 21 and are classified and, thereby, the monodisperse particles can be obtained.