PURPOSE: To allow the detection of oscillation through the output of an oscillation circuit changed with the oscillation of an oscillation plate, by locating a coil forming a resonance circuit in the oscillation circuit to an oscillation plate at least made of metal partly with a given distance opposingly.

CONSTITUTION: A detecting coil 2 is contained in a pot type core 21 through a given distance with an oscillation plate 1 made of metallic plate. The core 21 is fitted with a printed board 52 via an isolation sheet 51 and it is fixed in a case 5 with a resin 54 together with a circuit 53 to lead out a lead wire 55. The detecting coil acts like a resonance coil of the oscillation circuit 3. The distance with the coil 2 is changed through the oscillation of the oscillating plate 1 and the inductance is changed and the oscillated amplitude is changed for amplitude modulation. The output corresponding to the oscillation of the oscillation plate 1 is obtained through the output obtained by detecting it at a detection circuit 4. Thus, a detector having good frequency characteristics and being not subject to moisture effect can be formed.