To allow a differential electromobility classification system to be operated at the atmospheric pressure or below and to classify fine particles having specified particle sizes into particles having a uniform particle size by a method in which the projecting distance between a carrier gas ejection port and a classification part slit is decreased to improve sheath gas exhaust efficiency in an apparatus, an ultra-fine particle moving distance in classification is decreased to prevent the deterioration of resolution in Brownian diffusion.

Sheath gas 103 is introduced into a differential electromobility classification system, made uniform isotropically by a sheath gas buffer 104, and discharged from a sheath gas exhaust port 113 by way of a filter 118 and a classification part 109. Charged ultra-fine particles 119 are carried by carrier gas 107 from a distance- reduced carrier gas ejection port 110 by way of a slit part 111, exposed to an electrostatic field in the classification part 109, and turned in the axial direction of a cylinder according to the electromobility of the charge number and the particle size. Only the ultra-fine particles 119 which reached the slit part 111 in the lower part of an inner cylinder are taken out as classified ultra-fine particles 120 from a carrier gas exhaust port 115. In this way, operation is done at the atmospheric pressure or below, and classification is enabled for the ultra-fine particles 119 of 1-20 nm in particle size into particles having a uniform particle size.