The present invention discloses apparatus and method for rapidly and easily measuring the numbers and size distribution of low concentration particles which exist in a clean space such as a clean room. The apparatus and method according to the present invention measures the numbers of particles by charging particles to a monopolarity, collecting the particles by applying a voltage to an electrode and attaching the charged particles of a certain size or less thereto, separating the charged particles of the certain size or more according to size by the particle separating ducts. The apparatus for measuring the number of particles according to the present invention makes it possible to obtain a size distribution of particles in the air by a single measurement and to rapidly and easily measure it even though the number of the particles in the air is small.