PURPOSE: To reduce the number of parts constituting a circuit and simplify the structure.

CONSTITUTION: A magnetostrictive line is formed out of a magnetostrictive material and a current pulse generator 28 supplies a current pulse to the magnetostrictive line. A permanent magnet 30 is relatively displaced to the magnetostrictive line to make a static magnetic field cross the magnetostrictive line. A detecting coil 31 is arranged at a preset position on the magnetostrictive line. A detecting circuit detects a time difference between the current pulse and a magnetostrictive signal due to by mutual action of the permanent magnet 30 and the current pulse to find the displacement of a distance between the detecting coil 31 and the permanent magnet 30.