Provided is a device for assembling a fuel supply pipe of which a joint portion between a pipe body and a short cylindrical member has a high strength and which has high anti-corrosion performance. Specifically, while a laser beam in a defocused state is being emitted to impinge on an end portion of the pipe body that overlaps the short cylindrical member, the pipe body and the short cylindrical member are rotated through one complete turn relative to the laser beam, thereby melting the entire periphery of the end portion. While the laser beam in a defocused state is being emitted to impinge on the melted end portion, the pipe body and the short cylindrical member are rotated through at least one further complete turn, thereby joining by welding the end portion to the outer periphery of the short cylindrical member.