PURPOSE: To surely realize prescribed projection welding even when curvatures between both members do not always coincide at the time of the projection welding of a member with a curved recessed surface and a member with a curved projecting surface.

CONSTITUTION: The welding projection structure is constituted of two-stage projections consisting of support projecting parts 11 on which plural weld projections 10 arranging at proper intervals along the curve direction of a curved recessed surface 1b are formed on the curved recessed surface 1b in trapezoid shapes having top parts with proper areas and welding projecting parts 12 formed in tenon shapes, which project to proper heights and are provided successively on the top parts of the support projecting parts 11.