An object of this invention is to provide such a welded structure of and welding method for two metallic parts fitted together at a tubular section in a high-pressure fuel supply pump, that enables rapid joining of both metallic parts by staking (press-fitting included), screw fastening, and/or laser welding, offers high welding strength and sufficient fluid sealability, and keeps the metallic materials free from thermal changes in composition. Solution: While an electric current is supplied to any one of the welding connection surfaces formed between a pump housing and cylinder in a high-pressure fuel supply pump, between the pump housing and an installation flange, between the pump housing and an intake or discharge joint, between the pump housing and a pulsation-absorbing damper cover, between the pump housing and a relief valve mechanism, and between the pump housing and an electromagnetically driven intake valve mechanism, the particular two sections are pressed against each other to generate, along the connection surface, a plastic flow not causing melting or fusion due to heat, and form a diffusion-weld region at the connection surface by using the plastic flow and the application of the pressure.