PURPOSE: To provide star pole type or ring pole type arrangement of magnetic pole members at an attracting surface plate, by providing a fixed permanent magnet between the side wall of a container and a magnetism conducting member and the like, and rotatably inserting a rotary permanent magnet in a cylindrical cavity formed between a bottom plate and the magnetism conducting member.

CONSTITUTION: The upper side (side of an attracting surface plate) of a rotary permanent magnet 17 is of an N pole, and the lower side (side of a bottom plate) thereof is of an S pole. Then, a container 14 becomes an S pole and a disk shaped magnetism conducting member 16 becomes an N pole. As a result, an inner magnetic pole member 11 becomes an N pole, and an outer magnetic pole member 12 becomes an S pole. Therefore, the magnetic circuit of the attracting surface plate appeares on the attracting surface plate, and a material to be machined is attracted. Then, the rotary permanent magnet 17 is rotated by 180° by an actuating mechanism 19, so that the upper side becomes an S pole and the lower side becomes an N pole. As a result the magnetic circuit is shorted through the container 14 and the disk shaped magnetism conducting member 16. Then, the magnetic circuit does not appear on the attracting surface plate, and the material to be machined is released.