PURPOSE: To enable a position of an object which uses a conductor to be detected by converting an output signal of a bridge circuit which uses a solenoid coil as one side to a voltage and then calculating the position of the object.

CONSTITUTION: A detector 10 converts a high-frequency voltage output of a bridge circuit 9 to a DC voltage e1. This voltage e1 indicates a voltage which corresponds to a position in a solenoid coil 8 of an object 1. In an operation circuit 11, this voltage e1 is differentiated for calculating a speed change e2 with time of the object 1 and an acceleration change e3 with time, respectively. In this case, when the object 1 passes through a coil 8 where a pitch of a twisted wire changes at a constant rate with a locus 12 as a center line, a mutual inductance between the object 1 and the coil 8 changes depending on a position of the object 1. Therefore, a relation between the position of the object 1 and the change of the impedance is calibrated previously and an impedance change with time in the coil 8 when the object 1 passes is measured, thus enabling a position change with time of the object 1 to be detected continuously.