To solve the problem with a conventional piston head which is difficult to be joined since a projection is a plane or a substantial plane, and unable to be welded to a piston body as it causes separation due to insufficient joining.

A ring-shaped joining projection 3 is projected in a central part of a head member 2, and diffusively joined by butting mutual tops of the joining projection of the two head members. The joining projection is formed in a divergent shape, and the top is formed in a narrow plane shape. A foreign matter sticks to the outer periphery of a joining part of the diffusively joined two head members. In the head member, a joining projection is projected in a ring shape in a central part of its blocked-up surface. The joining projection is formed in the divergent shape, and the top is formed in the narrow plane shape.