To provide the dimension and the accuracy for a ball valve ball without any extra additional machining by molding a spherical part by rolling from a pipe material and by molding a ball valve ball.

Chamfer parts are formed in inside ends in both sides of a material formed by machining a stainless steel metal into a pipe so as to facilitate the molding of a bore part 3. Secondly, the material is set in the inside of a die 5 having a spherical face 4 in the internal circumference provided in a ring rolling machine and a mandrel 6 is provided in the internal circumferential position of the material. The ring rolling machine is driven, the mandrel 6 is revolved with rotation, at the same time upper/lower dies 5 are rolling machined while pressurizing in the opposite direction, and a ball 2 having a center-recessed internal circumferential spherical face 2a is pressurizedly molded into a shape along the internal spherical face 4 and the mandrel sphere part 7. This constitution can provide a prescribed circularity only by the molding by rolling and provide the dimension and the accuracy as the ball valve ball.