PURPOSE: To supervise whether transmitter is operating or not and the correctness of its transmission frequency by providing a rectifier which supplies operating power to a frequency divider and display device to these.

CONSTITUTION: The high frequency signal taken in by a pickup coil 11 is led into a shield case 12 and within this case 12, it is supplied to a frequency divider 13 constituted by complementary type insulated gate field effect transistors, e.g., C-MOSs. The high frequency signals having been divided down in this frequency divider 13 are amplified if necessary by an amplifier 14 and the amplified outputs thereof are supplied to a display device 15. From flickering of the display device 15, whether the transmitter is operating or not may be supervised.