PURPOSE: To surely detect only a sensing object without occurring malfunction even when an external noise intrudes by sending out sensing output only when plural frequencies inputted in a prescribed period show values exceeding a prescribed preset value.

CONSTITUTION: An oscillation circuit part 2 oscillates sequentially by the frequencies f1-fn. and oscillation output is inputted to a detecting part 3. Analog switches AS11-AS1n are energized corresponding to continuous waves, respectively, and each continuous wave is inputted to only a corresponding inductance calculation circuit 31, and is voltage-converted. A following detection circuit 32 issues no output by a voltage in accordance with inductance when no auxiliary capacitor is selected and no vehicle exists, thereby, it is outputted when a fluctuated voltage is obtained. Such output is converted to a pulse whose duration is extended by a signal extension circuit 33, and the output is inputted to a logical gate circuit 34, and is outputted when overlapped periods exist in each input. Therefore, the sensing output can be sent out only when the plural frequencies are fluctuated exceeding the prescribed preset value in the prescribed period.