PURPOSE: To present a radio data communication system which can predict and alarm the battery exhaustion of a slave equipment on the side of a host equipment.

CONSTITUTION: A timer part 5 measures time for communication with slave equipment 21-2n. A CPU 6 stores prescribed battery exhaustion time inputted through a key input part 7 in a memory 8, activates the timer part 5 in the case of starting the communication, successively reads the measured time of the timer part 5 during the communication, integrates the time for each slave equipment 21-2n and stores it in the memory 8 and when the integrated time exceeds prescribed time, a display part 9 is turned on.