To enable to assemble a device under a condition where a tip of a drain pipe is surely abutted on a bottom part of an air tank and make drain remaining in an air tank surly discharged.

This device is provided with a roughly straight drain pipe of which one end side is connected to a pipe joint 28 and of which another end extends toward a bottom part of an air tank 6, and a tip pipe 37 slidably installed on the drain pipe 30 and having a tip part thereof project outer than the drain pipe 30. The drain pipe 30 is formed in such length that another end side does not reach a bottom part of the air tank 6 under a condition where the pipe joint 28 is attached to the air tank 6. The tip pipe 37 is abutted on a bottom part of the air tank 6 at the tip part thereof when the pipe joint 28 is threadedly engaged to the air tank 6, and the tip pipe 37 is slid along a extension direction of the drain pipe with accompanying threaded engagement action of the pipe joint 28.The tip part of the tip pipe 37 get in a condition abutting on the bottom part of the air tank 6 when threaded engagement is completed.