PURPOSE: To obtain magnetic flux with ultrahigh density by covering a magnetic substance with a superconducting material cover and a superconducting material nozzle and narrowing down and converging the magnetic flux generated from the magnetic substance by the superconducting material nozzle.

CONSTITUTION: An electromagnet 2 is covered with the superconducting material cover 1 and the superconducting material nozzle 3 and the magnetic flux generated from the electromagnet 2 is led to the superconducting material nozzle 3 by the superconducting material cover 1 and the magnetic flux is narrowed down and converged by the nozzle 3 to obtain the magnetic flux with the ultrahigh density. The convergence of the magnetic flux can be carried out by the superconducting material and various products to apply this convergent magnetic flux can be produced. The alternating magnetic flux with the ultrahigh density is formed by converging the alternating magnetic flux in a spot-shape and the high temperature is obtained by applying this magnetic flux to a steel plate and it can be applied to the welding of steel plates.