PURPOSE: To make it possible to identify each individual by providing the difference between the times when individual identifying cards respond to calls from a ground station (master station) and start ansers, and sending the data from the individual identifying cards without the occurrence of interference.

CONSTITUTION: Individual identifying cards 2A, 2B, 2C...2N respond to calling signals (commands) I from a master station 1 and answer after times T1, T2,...Tn, respectively. When the transmission-state switching time is T1 and the transmitting time of the individual identifying time is TR, the time T2 is set at 2T1+TR, the time T3 is set at 3T1+2TR,... and the time TTn is set at nT1+(n-1)TR. Therefore, transmissions R1, R2,...Rn from the individual identifying cards 2A, 2B, 2C...2N are not overlapped, i.e. not interfered.