To provide a ball seat capable of effectively absorbing stress while reducing weight.

The ball seat 18 partitions a space 55 in a sealed shape between an outer surface 54 held in a socket and a sliding surface 53 slidingly contacting with an outer peripheral surface of a ball part. A portion of the ball seat 18 becomes a hollow shape, and the ball seat 18 can be reduced in weight, and can effectively absorb the stress such as when the ball part slides by elastic deformation of the space 55.