Abstract: A first magnetic scale consisting of a plurality of non-magnetic parts are arranged axially at equidistant intervals, and a second magnetic scale consisting of a plurality of non-magnetic parts arranged axially at unequal intervals, are formed parallel to each other on a rod consisting of a magnetic material which can be displaced in the axial direction. The non-magnetic parts of the second scale are symmetrically disposed on both side of a predetermined position on the rod. First and second magnetic sensors are disposed in fixed positions facing the scales.

Abstract: A magnetic scale is formed by a plurality of non-magnetic parts arranged at equidistant intervals in all axial direction on a magnetic rod which displaces in the axial direction. The non-magnetic parts consist of first non-magnetic parts having a constant depth, and second non-magnetic parts having a different depth. A magnetic sensor which outputs a signal according to the depth and positional change of the non-magnetic parts is installed at a fixed position opposite the magnetic scale. Further provided are a mechanism for identifying a second non-magnetic part which has passed the magnetic sensor based on the output of the sensor, a memory for individually storing the absolute positions of the second non-magnetic parts, and a mechanism for calculating the displacement of the rod from the identified second non-magnetic part to its current position based oil the sensor output.