To report the battery exhaustion of a personal computer(PC) data input device wirelessly connected with a PC.

A main body 3 of PC is connected with a PC a data input device 1 by the bidirectional radio communication with infrared rays or the like. The PC data input device 1 is driven by a battery 12 and the input operation of an operator is converted to an electric signal by a switch/encoder 14, converted to a specified code by a control circuit 15 and further transmitted through a modem part driving circuit 16 and a modem circuit 17 to the main body 3 of PC as a radio signal. The voltage of the battery 12 is detected by a voltage comparator circuit 13, inputted to the control circuit 15 as voltage data and transmitted to the main body 3 of PC in the same signal form as operation data. In the main body 3 of PC, this signal is received and converted to base data through a demodulation/modulation circuit 31 and a demodulation/ modulation part driving circuit 32 and these data are dispatched to the main body 3 of PC by a control circuit 33 and reported to the operator by an indicator 36.