PURPOSE: To accurately recognize the communication area of a radio transmitter without the need for a special measurement equipment.

CONSTITUTION: A control section 3 of a radio transmitter receives a call signal in common to all receivers from a memory section 2, sets an initial value as user data and sends the data for plural number of times. Succeedingly the control section 3 increments the user data and the same operation is repeated. When a reception section 5 receives a call signal and the user data, a control section 7 compares the user data of a memory section 6 with received user data and when the data are discontinuous, the received user data are overwritten with the user data of the memory section 6, an error message is outputted to a data output section 9 and the reception state is restored. When the received user data and the user data of the memory section 6 are the same or consecutive, the received user data are overwritten in the data of the memory section 6, the reception state is restored or after a normal message is outputted to a data output section 9, the user data are overwritten to the data of the memory section 6 and the reception state is restored.