PURPOSE: To detect the battery voltage of the transmitter side on the receiver side without adding any new constitution to the transmitter side.

CONSTITUTION: When a bicycle starts running, a rotation detecting part performs magnetic flux ratiation, taking the rotation of a magnet fitted to a front wheel as an electromagnetic induction signal. Whereupon, electromotive force is generated at both end parts of an electromagnetic induction coil 50 of a receiving part 40 to receive an electromagnetic induction signal. The electromagnetic induction signal is amplified by a detecting circuit 52, an electromagnetic induction signal component exceeding a threshold is taken out as a rectangular wave (pulse) by a comparator 53, and the number of pulses is counted by a pulse processing circuit 54. The received electromagnetic induction signal, the peak voltage value of which is held by a peak hold circuit 55, is converted to digital data by an A/D converter 56. CPU 41 displays shortage of power of the transmitter side if the counted number of pulses is below a predetermined number or the peak voltage value becomes less than a predetermined level.