In the prior art, only the case where the contact is made on the stabbing flank face 31B is estimated and hence, the prior art has a drawback that when a highly concentrated load acts on the load flank face 30P of the threaded portion on the pin pipe end side, the concentrated load cannot be decreased. In a premium joint, a pitch change is made such that an internally-thread pitch 20B has the same value as an externally-thread pitch 20P from a thread end portion on a box 1 end side to a pitch change position (PCP) taken on a box center side with respect to a position away from the thread end portion by 1/2 of an entire length of the threaded portion, and the internally-thread pitch is changed to a value less than the externally-thread pitch at the pitch change position (PCP) so that a thread gap 10 is gradually decreased from the pitch change position to a position (PCP) of the thread end portion on the box center side, and the thread gap is eventually set to a value which is 0.2 to 0.8 times as large as the thread gap 10 before the pitch change is made.