PURPOSE: To detect the proximity of a metallic substance at a high speed.

CONSTITUTION: The output of an oscillation circuit 1 which has a fixed amplitude with a fixed frequency is given to a filter circuit 2 which contains a detection coil L as a filter element. The inductance and the resistance of the coil L vary with proximity of a metallic substance and therefore the filter characteristic also varies. Thus the variation of the output passing through the filter is detected by a voltage comparator 3. When this output continues, a substance detection signal is outputted from an output delay circuit 4.