PURPOSE: To improve the strength after joining by consisting the complex solder of solder which consists essentially of tin and lead, a metallic reinforcing material, and a flux which accelerates wetting of both, subjecting the solder, the reinforcing material and the flux to mixing and stirring, and melting the mixture by heating, thereby producing the composite solder.

CONSTITUTION: Paste 2 consisting of powder solder 21 made of Sn-Pb and the flux 22 which accelerates wetting of the powder solder to metallic particles 3 and the metallic particles are mechanically mixed and stirred at room temp. The pasty material mixture in which the metallic particles are uniformly dispersed in the powder solder 21 and the flux is stuck to the surface of the metallic particles is produced. The pasty material mixture 4 is mounted between the joint surface 51 of a metal 5 and the joint surface 61 of a metal 6. The pasty material mixture is heated to melt the powder solder. The flux sticking to the metallic particles accelerates the formation of an alloy layer 25 consisting of the compd. (Ni-Sn or Cu-Sn) of the metallic particles and the molten solder 24. The solder and the reinforcing material are made into the composite material in this way, by which the strength after joining is improved.