The present invention provides a fine-
particle counter with which the 
number density of nanometer-sized fine particles born in a 
gas phase, which is extremely low, can be accurately measured under wide-
ranging pressure conditions from pressurized conditions to low-pressure conditions.After contact-mixing, in a mixer 3, saturated vapor of a high-boiling-point 
solvent produced in a saturator 2, a component of a condensed 
nucleus detector 1, with nanometer-sized fine gas-born particles, condensed droplets of the saturated vapor whose nuclii are the fine particles are produced in a condenser 4 by heterogeneous 
nucleation. The number of the condensed droplets per 
unit of time is then counted with an 
optical detector 5 and is output as a pulse 
signal, and a computer 19 computes the 
number density of the nanometer-sized fine particles born in the 
aerosol from this pulse 
signal, the gas flow rates controlled by the flow meters 6, 12 and 10, and the other data that are transmitted to the computer 19 via an interface 18. The internal space of the mixer 3 has a narrowest passage having a circular cross section, situated in the center between the lower end of the mixer from which the carrier gas enters and the upper end of the mixer from which the carrier gas exits, a truncated-cone-shaped part whose cross section is circular and whose 
diameter gradually decreases so that the 
diameter on the lower end side is greater than the 
diameter on the narrowest passage side, and a reverse-truncated-cone-shaped part whose cross section is circular and whose diameter gradually increases so that the diameter on the narrowest passage side is smaller than the diameter on the upper end side. An 
aerosol inlet communicating with the 
aerosol inlet tube 8 is positioned at the narrowest passage.