First steel material, a carbon sheet, and second steel material are put in a state of being separated from each other, and bonding parts are heated by, for example, applying current from a power supply to the carbon sheet. Alternatively, the bonding parts may be heated by an inductive heating coil. Thereafter, heating is terminated if the bonding parts reaching greater than or equal to the eutectic point and less than the liquidus line temperature is detected in the Fe—C phase diagram. Furthermore, the carbon sheet is made to be sandwiched between the first steel material and the second steel material.