PURPOSE: To surely recognize the respective presence of plural objective bodies moving irregularly by extracting respective identification codes when high-frequency signals from a slave device is received at a master device and displaying them at the corresponding position of a display part when they are matched to a prescribed standard.

CONSTITUTION: When radio waves modulated by a classification code are received from the master device, the slave device transmits the radio waves modulated by the classification code and the objective body numbers in a cycle determined by random numbers when the classification code of a decoded result coincides with a set value. At the master device, the radio waves from the slave device are received and detected by a tuner 202, the output is decoded by a decoder 203 and the respective identification codes are extracted. Then, rationality checking by a check bit or the like and collation by the classification code are performed at a collation circuit 221. The recognized respective identification codes are supplied to a controller 206 and a display control part 210 is instructed to identify the objective body number and to turn the pertinent position of the display part 211 into a display state.