PURPOSE: To assemble by caulking two or more members differing in material and heat treatment method without being influenced by thermal effects, by pressurizing the base metal supported by a lower electrode with an upper electrode provided with coolant passage, and, after resistance heating and caulking with increased pressure, quenching the upper electrode to temper by temper current.

CONSTITUTION: An SPC lever 2 is carburized and hardened, and is fitted to a head part 1a of a shaft 1 which is the base metal. The shaft 1 is vertically held by a lower electrode 5 which is cooled by a coolant, and an upper electrode 4 is caused to abut vertically against the head part 1a, and the pressure is initially increased to point A. At point B, then, a current is passed into the electrodes 4, 5 from an electric resistance welding machine power supply to increase the pressure to point C while the pressing force of the electrode 4 is increased to point D to caulk the head part 1a by resistance heating. Then, keeping the pressing force at point E, power supply is stopped at point F, and the coolant is supplied into the passage 4a to quench the electrode 4 and cool the head part 1a. Next, the electrode is tempered by passing a temper current between the electrodes at point G, and power supply and maintaining of pressing force are stopped at points H and I.