A flange of a first pipe and a flange of a second pipe are connected via the flanges. An annular seal groove is formed in the flange surface of the flange. A sealing liquid is poured and placed in the seal groove from a sealing liquid pouring section via a pouring passage. The sealing liquid placed in the seal groove forms a liquid seal structure to improve sealing ability. Further, the sealing liquid is placed also in a discharge passage and a discharge pipe. When gas in the pipes enters the seal groove, the gas can be discharged from a discharge device and detected by a gas sensor to enable the user to take safety measures.