The laser irradiation step includes: removing zinc plating layers respectively disposed on the surfaces of the galvanized steel sheets by the laser irradiation; and melting the surfaces of the galvanized steel sheets of which the zinc plating layers have been removed, and the roller pressurizing step includes pressurizing the galvanized steel sheets by the rollers while the surfaces of the galvanized steel sheets that have been melted are in contact with each other such that the galvanized steel sheets are joined together.