PURPOSE: To discharge periodically and forcibly wet air having been taken into a piping by providing a safety valve which is actuated above an ordinary pressure and an outlet port before a loading port of a device into which air is fed.

CONSTITUTION: Air which has not been sufficiently dried at a moisture absorbing tank 6 of a piping is periodically discharged to the outside from a discharge port 24 through a safety valve 23 provided before a loading port 17 to a device into which the air is fed. Therefore, without disintegrating joints of pipings, dry air can be always supplied, while wet air does not remain in an air pressurizing piping and a device into which the air is sent.