PURPOSE: To solve the sealing problem of causing a scratch or a little deformation in a sealing surface by sealing a wellhead, a blowout preventer and a production tree with a tapered sealing ring to form a joint.

CONSTITUTION: A joint 10 is formed by a lower member including a wellhead housing 12 having a center hole 14 and an upper member including a blowout preventer erect cylinder 16 having a center hole 18. A sealing ring 52 which is stored in the center hole 54 and recessed parts 30, 42, and has a hub 56 engaged with faces 34, 46 is closely engaged so that the hub 56 is clamped by the faces 34, 46, and the respective faces of the tapered faces 58, 60 reach Brinell hardness. After excavation, a blowout preventer erect cylinder 16 is detached and a production tree 66 is fitted to the upper end part of the housing 12 by the same method. Thus, the near joint member sealing surface can be protected.