Abstract: In a single seat valve apparatus, a valve box has upstream and downstream fluid flow channels. A cage is fixed in the valve box and has a flow rate control window for controlling an amount of the fluid flowing from the upstream to the downstream flow channel. A valve seat is provided to the cage. A valve body is slidably fitted in the cage, closes the control window, when seated on the valve seat, to block a passage of the fluid from the upstream to the downstream flow channel, and forms a passage of the fluid, when separated from the valve seat, from the upstream to the downstream flow channel through the control window. An opening is formed in the valve body in an axial direction of the cage and allows a fluid space and a space on one side in the axial direction of the cage to communicate with each other, to equilibrate a pressure applied by the fluid. A valve stem is connected to the valve body and moves it in the axial direction of the cage.