To strengthen a jointed condition between a valve casing main body and a valve seat casted and jointed to the valve casing main body.

While a valve casing main body 1 is molded under a condition that a filler metal in which a first ring body 4 for a valve seat and a second ring body 5 which is fixed to the inner circumferential surface of the first ring body 4 so as to prevent the distortional deformation of the first ring body 4, are overlapped is set in a core, a pivotal part on the peripheral surface side of the first ring body 4 is casted and jointed to the inner circumferential surface of the valve casing main body 1 so as to manufacture a valve casing B equipped with the filler metal, and after that, the projected part 7 of the first ring body 4 and the second ring body 5 are cut off so as to form the valve seat so that a valve casing in which the valve seat is casted and jointed to the inner circumferential part of the valve casing main body 1 is manufactured.