To measure precise particle size distribution with a narrow particle size band width set therefor and change of particle size with time with a wide particle size band width set therefor, with one instrument, in an apparatus and an instrument classifying particles and measuring particle size using particle size dependency of electrical mobility of charged particles.

In an outlet slit 13 for selectively taking out the charged particles, an opening width in the sheath gas flow direction in which charged particles with different particle sizes are separated according to the slide position of a shielding plate 21, is set adjustable.