The unsteady motion of small particles, which are uniformly distributed at first, in a square chamber with a pulsating inlet velocity is studied numerically. The force between particle and fluid is assumed to be proportional to the relative velocity. The problem considered is governed by four parameters, the Reynolds number, the dimension ratio (which characterizes the particles as coarse or fine), the period and the amplitude of the pulsating inlet velocity (which are made dimensionless). The motion of the particles is classified into three types : the exhaust type, in which the particles are exhausted through the outlet ; the circulation type, in which the particles circulate in the chamber ; and the adhesion type, in which the particles adhere to the wall of the chamber. The effects of the period and the amplitude of the inlet velocity on the ratio of the number of particles of each type to the total number of particles are examined taking the Reynolds number and the dimension ratio as parameters. The suitable ranges of the period and the amplitude of the inlet velocity, in which the number of exhausted particles becomes large, and the number of circulating particles becomes small, are found, taking the dimension ratio as a parameter. The mechanism of the exhausting of the particle is qualitatively clarified on the basis of the relationship between the streamline and the trajectory of the particles.