PURPOSE: To improve the efficiency of an exchange program by detecting a dial pulse with a low-speed period program.

CONSTITUTION: When a signal R attains an H, a call request is sent to the exchange program. When the signal R varies from the H to an L in a dial pulse detecting circuit while a signal E is at the H, a signal STB is held at the H through a circuit 3 to place a circuit 1 in a ready state, and the calculation of the circuit 1 is performed every time the signal R varies from the L to the H to output a dial pulse one after abother as parallel binary data to a signal group DP. A circuit 6 as a minimum pause detecting circuit operates after the signal R is sent by one digit of the dial pulse and a signal TM1 attains the H. The exchange program only detects the signal group DP when the signal TM1 is at the H, and the circuit 6 is so configured that the signal TM1 is at the H continuously for 250ms; the dial pulse detection program of the exchange program is only to detect the pulse with the program having a ≤200ms period.