To improve joining strength of a valve seat by forming an annular projection streak which is projected on the inner circumferential side of an opening part on the valve seat sitting surface of a cylinder head main body, and forming a valve seat matrix so as to form an outer peripheral surface which is inclined in a specified direction and a bottom surface respectively.

In a valve seat, a valve seat matrix 20 is arranged by joining with a valve seat sitting surface 40, and the valve seat sitting surface 40 is formed of a flat surface 41, inner side taper surfaces 42, 43, and an outer taper surface 44. An annular projection streak 46 which is projected on the inner circumferential side of an opening part 13a is formed by two surfaces of the flat surface 41 and the inner taper surface 42. An outer peripheral surface 50, a bottom surface 51, an inner circumferential surface 52, and an upper surface 53 are formed in a valve seat matrix 20, the outer peripheral surface 50 is inclined so as to form an outer diameter narrower gradually as approaching to a cylinder head main body 11 side, and the bottom surface 51 is inclined so as to unevenly distribute it more gradually as approaching to an axial core.