A pair of thermoplastic resin members are placed on an anvil. A pressing force of a tool horn vibrating ultrasonically in a direction not perpendicular to but along upper surfaces of the pair of thermoplastic resin members is applied to the upper surfaces. The application of the pressing force of the tool horn vibrating ultrasonically allows melting of a vicinity of the upper surfaces of the pair of thermoplastic resin members. A welded structure part is formed on an unwelded structure part, thereby welding the pair of thermoplastic resin members as an overlap structure including the welded structure part arranged on the unwelded structure part. The distance and positional relationship between the pair of thermoplastic resin members after the welding are unchanged before and after the welding. The surfaces, placed on the anvil, of the thermoplastic resin members are neither burned nor discolored.