To protect an ignition material and a thermit material in a crucible from water or oil by constituting the inside of the crucible formed of porous ceramics to be liquid-tight.

A thermit welding tool comprises a cylindrical crucible with a bottom having a penetration part, and a thermit welding tool body part. A glass layer or a metal layer is deposited on the inner or outer surface of the crucible. The thermit welding tool body part comprises a cavity part forming a welding part and a passage communicating with the penetration part and the cavity part. The crucible is cartridge-shaped and is made freely attachable.