A first MR sensor and a second MR sensor are a combination of a first magnetic resistance effect element pattern and a second magnetic resistance effect element pattern. The first MR sensor and the second MR sensor are disposed a prescribed distance apart such that, when the first MR sensor receives the greatest quantity of the magnetic field component of a magnet oriented parallel to the axial direction of a piston, the second MR sensor receives the greatest quantity of the magnetic field component of a magnet oriented parallel to the radial direction of the piston.