PURPOSE: To obtain a tennis ball into which air and gaseous LP are can be injected even if the internal pressure of the hardball tennis ball drops.

CONSTITUTION: A plug 1 is fitted into the middle side of the ball and the plug 1 is provided with an injection port 2, its cap 4 and an outlet hole 3. A cap 6 is put on the plug 1 and closes the output hole 3. The cap 6 is adhered at the root part to rubber 7 of the ball so as to prevent the disengagement of the cap from the plug 1. To feed the air into the ball, a user inserts the nozzle of an air pump for bicycles into the injection port 2 and injects the air little by little into the ball while checking the internal pressure of the ball with his hand. The air and gaseous LP are injected again into the tennis ball which is heretofore unusable in one week by the drop of the internal pressure by 9 to 21%. The semipermanent use of the ball by increasing the internal pressure is thus possible.