To enable a bung 4 to be melted in a short time only at its outer surface when the bung 4 to be welded to a container 2 made of a resin is previously melted by heating.

The bung 4 is held at a bung holding means 10 provided at an index wheel 6 and intermittently conveyed. A contact-type heater 16 is provided at a first heating position B, and is brought into contact with a fusion bonding part 4a of the bung 4 and heated to a temperature (130°C) lower than a melting point (e.g. 140°C) of a material of the bung 4. A non-contact-type heater 22 is provided at a second heating position C, and the bonding part 4a is heated at a temperature (320-330°C) higher than the melting point without contact with the bung 4. The part 4a of the bung 4 can be heated to the vicinity of the melting point in a short time by the heater 16, but is not melted, and hence a whisker-like projection is not formed, and the part 4a can be heated to the melting point or higher by the heater 22, and can be melted in a short time.

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