Abstract: The reason, condition and rule of the magnetic leakage field yielded by a subsurface artificial hole in the tool steel ring specimen over the ring surface at circular magnetization by direct current are analyzed in the present paper through the concept of magnetic dipole chain. It is proved that this magnetic leakagefield is a multivariant function of the ring diameters, the hole diameter, the buried depth of the hole, the circumferential magnetizing current, the saturated magnetizing current and the magnetic susceptibility of the steel ring etc.