PURPOSE: To very the length of the transmission signal in accordance with the amount of information to be transmitted by decoding the contents of each signal of the received signal group and deciding the length of the data signal.

CONSTITUTION: The signals are transferred between a fire receiver 11 and the main receivers 12-14 via a main loop signal line 21, The data type signal showing the contents of DATA shows the information on the state, the information on the test, the information on the control, and the information on a database respectively. Then an even byte of an optional and size larger then the size of the data base is assigned to the database information in response to the database to be returned. The receiver side of the transmission signal decodes the data length signal included in the received signal and discriminates those information on the state, the control and the database respectively.