PURPOSE: To perform desired measurement even when the distance between two bodies is relatively long by disposing a short circuit coil at the other part of an equipment coupled with the other body.

CONSTITUTION: One body moves with respect to the other body such that the intervals between a short circuit coil 60 and coils 50, 52, 54 are kept constant. More specifically, the insertion depth is varied but the interval 64 is kept constant. Alternatively, the coils 50, 52, 54 may be arranged on two bodies such that not the insertion depth but the interval between the surface of the coil 60 and the coil 50 and/or the coils 52, 54 are varied when one body slides on the other body. In this regard, first and/or second measuring voltage is varied and an electronic circuit 76 can detect the interval between two bodies mutually. Consequently, desired measurement can be carried out even when the distance between two bodies is relatively long.