Abstract: First and second ridge portions at the foremost end of a flexible pipe are pressed into first and second enlarged-diameter ridge portions, respectively, each having an external diameter larger than the internal diameter of a rotary nipple and smaller than the front-end root diameter of a taper external thread of the rotary nipple. The back surface of the second enlarged-diameter ridge portion is brought into contact with a front-end surface of the rotary nipple. A rubber seal ring is fitted into each of a first and second root portion of the flexible pipe, respectively, and an aligning ring is fitted into each of a third root portion of the flexible pipe located on the front-end side thereof from the first root portion and a fourth root portion of the flexible pipe located on the back side thereof from the second root portion, respectively.