Abstract: The known magnet plug in which a cylindrical permanent magnet is fitted in and secured to a recess formed in a top surface of a threaded plug main body, is improved so that the permanent magnet can be firmly secured to the plug main body without damaging the threaded portion of the plug main body nor without magnetizing the threaded portion by the plug main body. The improvements reside in that a hollow cylindrical piece having a smaller outer diameter than that of the threaded portion of the plug main body is projected from the tip end surface of the threaded portion, a permanent magnet piece having an annular step formed on its outer peripheral surface has its bottom portion fitted in the hollow cylindrical piece, and the permanent magnet piece is fixedly secured to the plug main body by shrivelling the entire peripheral wall of the hollow cylindrical piece into the annular step.