PURPOSE: To improve the reliability of charging information detection by reading the contents of an ROM by a control circuit and registering and displaying abnormal charging information distinctively from normal charging information.

CONSTITUTION: A charging signal and an abnormal charging signal (a) sent out of the exchange 8 of an office are converted into signals (b) having a logic level when coincident with a frequency determined by a frequency detecting circuit 2. The control circuit 4 reads the contents of the memeory to start a start period decision program, charging signal detection program, charging signal information, and abnormal charging information registration program periodically, and abnormal charging information is registered in the control part 7 of a main body and also displayed distinctively from normal charging information.