PURPOSE: To further surely achieve the sealing before the leakage of a damaged sealing mechanism is prevented by injecting the sealant as the final means.

CONSTITUTION: Inner and outer sealing parts with a ball 9 are provided by a first seat 14 which is held by a first seat retainer 11 and backed up by a first outer ring 33 and a second seat 10 which is held by a second seat retainer 7 and backed up by a second outer ring 30, a sealed chamber 16 provided between the second seat retainer 7 and a stepped part 15 of a body 3 communicates with the atmosphere to detach the second seat 10 from the ball 9, and the sealed chamber 16 communicates with a flow passage part 4 to bring the second seat 10 into contact with the ball 9. The sealant is injected into the pace 28 between the ball 9 between the first seat 14 and the second seat 10 and the first and second seat retainers 11, 7 from the outside.