Abstract: 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.