To lock a heavy material such as a white board, a frame or a glass mirror to its surface on an object such as a steel panel or steel furniture which is difficult to be fixed with nails and screws.

This planar magnet body comprises a planar base and a single or a plurality of neodymium magnets incorporated in the planar base. One surface of the planar base is provided with an adhesive layer or a pressure sensitive adhesive layer and a releasing paper covers over the surface of the adhesive layer or the pressure sensitive adhesive layer.