To provide a pipe joint holding air-tightness all the time.

A tip end of a plug P fixed to a joint J by a bearing ball B is pressed to a gasket G, and leaking of a fluid flowing in the inside of the plug P and the joint J is prevented by the gasket G. A fluid is supplied to a cavity Ah, an air seal A is expanded, and a clearance between the joint J and the plug P is completely filled with the expanded air seal A. In this way, a fluid flowing in the inside of the plug P and the joint J can be completely interrupted from the outside. A pressure switch PS and a safety valve SV are provided in a flow path of fluid flowing in the air seal A. A pressure in the air seal A is adjusted by the pressure switch PS and the safety valve SV.