A pin includes, in order from a free end thereof, a shoulder surface, a first sealing surface, a first male threaded portion, a second sealing surface, and a second male threaded portion. A box includes a shoulder surface, a first sealing surface, a first female threaded portion, a second sealing surface, and a second female threaded portion. The flank angles of load flanks of the first male threaded portion and the first female threaded portion are each less than 0 degrees. The first male threaded portion is composed of, in order from near the second sealing surface, an incomplete thread section and a complete thread section. The incomplete thread section has a length of at least three times the thread pitch thereof and has a thread height lower than the thread height of the complete thread section. The incomplete thread section is provided with clearances between crests and roots.