A plurality of Hall ICs 34A, 34B are disposed around a magnetic rod 32 that is movable along a central axis. The Hall ICs are disposed in different positions in terms of both a straight line distance coordinate in the direction of the central axis and a rotational angle coordinate around the central axis. The amount of displacement of the magnetic rod 32 is calculated based on the average of output signals from the Hall ICs. Errors in the output signals due to a shift and tilt of the magnetic rod are detected and calibration of the measurement method is carried out.