To provide a particle counter, for the leak test of an air filter, in which a bad influence is not generated after the leak test and by which a high- accuracy leak judgment can be made surely.

A particle counter for a leak test is of a type by which a probe 1 is scanned along the downstream side face of a filter medium at an air filter so as to detect the existence of a leak in the filter medium. In the particle counter, the number of particles from a clock signal TA in a present point of time up to a next clock signal TA+1 is detected by a particle detection part 2 connected to a probe 1 so as to be counted by a particle counting part 3, its counted number is stored in a memory part 4, and the number of particles counted between the clock signal TA and the clock signal TA+1 and the number of particles stored and held by the memory part 4 from the clock signal TA up to a clock signal TA-N before a definite time from the signal are added by a number-of-particles recomputing part 5.