Via a first path (P1) which extends for continuous treatment from one longitudinal end to the other of a cylinder head (H) passing between pairs of exhaust ports (15) and pairs of intake ports (14) opposed to each other with respect to one cylinder and which terminates in a tension bolt hole (21), and via second paths (P2P5) which, after treatment by this first path (P1), extend for successive treatment from cylinders adjacent the terminal point of the first path (P1) and between a pair of exhaust ports (15) and a pair of intake ports (14) opposed to each other and which terminate in tension bolt holes (21), the surface of the cylinder head (H) is stirred for refining without melting it by the heat from a rotating tool.