PURPOSE: To perform data transmission suppressing power consumption by measuring a distance between the master device on the side of reception and one of the slave units on the transmission side and performing the signal amplification according to a propagating distance.

CONSTITUTION: When a command issuing instruction is sent from a CPU 2, both modulation control signals sent from an SIO3 and signals sent from a sender 7 are inputted and modulated to a signal modulator 8. A signal amplifier 9 amplifiers the modulated signal in the output adjusted according to a propagating distance by an output adjusting device 11 and then it is sent from a transmission module 10. The transmitted signal is received by a reception module 14 of the slave device not shown in a figure and amplified by a signal amplifier 15, then selected by a bandpass filter 16 and formed by a waveshape forming device 17, further inputted to the SIO3 and command-processed by the CPU 2. Thus, the data can be sent by the output according to the propagating distance and a power consumption can be suppressed.