PURPOSE: To make it possible to automatically adjust the supply amount of air into a pressure chamber, by providing such an arrangement that a valve member is raised by a motor-driven pump which is started when the amount of air in the pressure tank becomes greater than a predetermined value, and therefore, a through-hole which is communicated with a communication pipe led to the pressure tank of supply air, is closed.

CONSTITUTION: A first valve member 11 is further raised in an upper valve chamber 4 when air in a pressure tank 1 of supply air comes up in a large amount through a pipe member 10, and therefore, depresses and closes the opening of a through-hole 9a which is communicated with a communication pipe (not shown) led to the pressure tank 1. Further, when a motor-driven pump is started during the above-mentioned operation, the suction side of the pump becomes negative so that the first valve member 11 is rapidly raised to instantly constant and close the opening of the through-hole 9a, and as well the closed condition is sustained by the surface tension of an air bubble. Thereby, the supply of air from an automatic air supply mechanism 13 to the pressure tank 1 is instantly cut off. As a result the supply amount of air into the pressure tank 1 may be automatically adjusted, and as well, an excess of air supply may be prevented.