A transmitter portion 14 includes a control circuit 32, a driver circuit 33, and a wave detector circuit 35. The control circuit 32 generates control signals Vp, Vn based on a pulsed transmission signal So. The driver circuit 33, which is supplied with a battery voltage Vb, generates an output signal Sa to be sent to an antenna 13 based on the control signals Vp, Vn. The wave detector circuit 35 outputs a detection signal Sk having a voltage proportional to the level of the output signal Sa (transmission power level). The control circuit 32 changes the pulse widths of the control signals Vp, Vn based on the voltage level of the detection signal Sk. As a result, electric power consumption is reduced and enlargement of an apparatus is suppressed while maintaining the transmission power level constant.