To secure high reliability and safety of the welding by performing the thermite reaction in a crucible made of α-Al2O3 of the prescribed density and bulk density.

The thermite reaction is performed in a crucible made of α-Al2O3 of porous or hollow spherical shape. α-Al2O3 is 0.8-3.0g/cm3 in density and 0.3-1.8g/cm3 in bulk density. More preferably, the reaction is performed in the crucible of 1.0-2.0g/cm3 in density and 0.4-1.4 g/cm3 in bulk density. For this purpose, e.g. the porous corundum which is manufactured by melting the calcinated alumina and blowing air into the melt is preferable. Workers can be protected thereby.