To carry out highly precise measurement with excellent linearity by providing a non-magnetic container which contains a spherical body made of a magnetic body in the inside and has a spherical space having larger diameter than that of the spherical body and magnetic poles installed at positions, which are points of contact of the spherical space and a cubic body circumscribing the spherical space.

A spherical body 1 is a magnetic body and made of an iron ball. A container 2 has a spherical space to contain the spherical body 1 and is made of a non-magnetic synthetic resin. A diameter of the spherical space of the container 2 is made wider than that of the spherical body 1, so that the spherical body 1 can move in the container 2. Magnetic poles 3-8 are installed at positions which are the points of contact of the spherical space and a cubic body circumscribing the spherical space. Coils 13-18 are coiled on respective magnetic poles 3-8 and a.c. current is applied to one side of each coil as an excitation side and the induction voltage increases more in the other side, which is a detection side, as the spherical body 1 comes closer. The magnetic poles 3-8 are connected with a yoke 10 made of a magnetic body and the yoke 10 works also as a magnetic shield. Of the coils, coils 13 and 14, coils 15 and 16, and coils 17 and 18 are differentially connected, respectively.