To hold fixed internal pressure at all times by providing a check valve provided with an injection port for injecting air and a discharge port on the inner wall of a ball and injecting the air to the ball by an air injector with a nozzle.

In the process of manufacturing the ball, before adhering two rubber semi-spheres in the case of the hard tennis ball and before adhering the two or three sheets of rubber in the case of the soft tennis ball, the check valve 2 provided with the injection port 1 for injecting the air and the discharge port 3 is attached to the inner wall of the ball by an adhesive material. The check valve is provided with elasticity like the rubber of a rubber balloon and is closely adhered to the inner wall of the ball. The air injector is constituted by attaching the nozzle instead of the needle of the well-known air injector with the needle and the air is injected inside the ball after putting the nozzle in the injection port 1. When the air is injected too much, by slightly pushing up the check valve 2 through the injection port 1 and discharging the air to the outside of the ball, the internal pressure is appropriately adjusted.