PURPOSE: To avoid the momentary effect of a radio line and to obtain a highly reliable data by providing a detection section detecting the presence of an answer signal from a slave station to a master station and allowing only a slave station whose answer signal is missing to apply answerback operation again automatically.

CONSTITUTION: An answer signal designated from slave stations 3-5 receiving an answer request signal from a master station 1 is returned in a radio wave 2. A transmitter-receiver 6 of the master station 1 receives the radio wave via an antenna 7, the output is processed by a data processing section 9 and a detection section 12 detects the absence of the answer signal of each slave station. If there is any missing in the answer signal, a number of an object slave station is outputted to an answer request setting section 8 to apply answerback operation only to a slave station whose answer signal is missing automatically. Thus, the missing of data due to momentary disturbance of a radio line being a transmission line is prevented to obtain a highly reliable data.