PURPOSE: To prevent the generation of a welding defect in a piston, by directing the welding face of an embedding plug toward the inside of a cooling oil passage below the embedding plug in the piston in which the embedding plug is fitted and welded by an electron beam to an annular groove formed to the upper face in the heat part of the piston.

CONSTITUTION: An annular groove 2 is formed to an upper face in the head part of a piston 1, and its bottom part is applied with boring work to spread a width. Then an embedding plug 3 is fitted to an upper part of the groove 2 to close the groove 2 by electron beam welding in the direction of arrow heads f1, and a cooling oil passage 4 is formed by a closed space below the plug 3. Here the plug 3 is formed narrower than the passage 4 to direct a welding face g1 of the plug 3 toward the inside of said passage 4. In this way, a space is formed ahead from the bottom end of the face g1 of the plug 3 with no metallic wall existent, at welding of the plug 3, a melting pond is never generated ahead from the bottom end of the face g1. Accordingly, a welding defect is never generated in the piston 1 and its strength and durability can be improved.