PURPOSE: To surely collect data of mobile slave station by sending data to a slave station able to be communicated with a master station through a standby time slot from a slave station outside of a communication enable area and allowing the slave station to make relay transmission to the data.

CONSTITUTION: A master station 100 sends a data request signal (a) through a radio circuit 110. A slave station 201 receives the signal (a) through a radio circuit 210 and sends data (b) to the master station 100 through a designated time slot. On the other hand, a slave station 202 unable to receive the signal (a) by an obstacle 300 receives the data (b) sent from the slave station 201 to the master station 100 by the circuit 210 and discriminates it that a data transmission request comes from the master station 100 and sends data (c) to the slave station 201 through a standby time slot. Thus, the slave station 201 relays the data (c) of the slave station 202 and sends the data even at a point at which the slave station 200 cannot directly communicate with the master station 100 to allow the master station 100 to surely collect data.