A hot diffusion welding method wherein a first part generally made from a material different from a second part and the second part made from copper alloy are united with an insert material interposed between the joint faces thereof under a heated and pressurized condition. The insert material at the joint between the first and second parts is copper plating formed on the joint face of the first part made from the different material, so that the thermal conductivity between the two parts is improved to perform smooth resistance heating. This enables continuous welding operation without polishing the conductive surfaces of a pair of electrodes in order to eliminate oxides therefrom.When the insert material interposed between the joint faces of the two parts is a normal brazing filler metal, resistance heating is carried out with an even number of superposed bodies in each of which the second part, insert material, first part are superposed, the superposed bodies being assembled in such a manner that the respective second parts are brought into contact with each other. In this case, the first parts having a greater thermal capacity are in contact with the pair of electrodes, whereby polishing for eliminating oxides from the conductive surfaces of the electrodes is no longer necessary and continuous welding operation can be thus accomplished.