PURPOSE: To reduce the power consumption for communication between vehicles on road by using an induced electromotive force with a loop coil for vehicle sensing as an operating power supply for a transmitter.

CONSTITUTION: When a vehicle detects a vehicle on a vehicle sensing loop coil 1, a vehicle sensing signal is taken out from a phase detection circuit 20 through the inductance change and outputted. While the vehicle is sensed, the coil 1 outputs a frequency signal f1. When the transmitter A receives the frequency signal f1, the on-vehicle loop coil 11 oscillates a frequency signal f2 to a round equipment in response to a vehicle data signal. When the ground equipment receives the frequency signal f2, a demodulation data is outputted to a central equipment from a control circuit 28 via a modulator 29. To a vehicle not mounted with the transmitter A, the frequency signal f1 is rejected by a band pass filter 26 and only the vehicle sensing signal is outputted from an output circuit 21.