PURPOSE: To return the changes from a 100 yen public telephone by using a charge clearing system and clearing the changes at the time of using the 100 yen public telephone.

CONSTITUTION: The changes control system of a public telephone has a 100 yen public telephone set 1, a subscriber line 2 to link the 100 yen public telephone set 1 and an exchange 3, data link 4 to link the exchange 3 and an accounting processing system 5, the accounting processing system 5 and a list 6 for requiring a telephone charge. The accounting processing system 5 calculates the number of the times of calls by a ticket received in a ticket receiving processing 14, that is, a changes calculating processing 15 and a charge clearing output 16, calculates the changes, the result is subtracted from the general telephone charge of a contract subscriber and the telephone charge is cleared in units of one month.