Screw thread

A screw thread that is configured for insertion in an internal screw thread, comprising:

• a recessed portion on a flank surface of a screw thread ridge provided on a screw thread shaft main body, wherein
• the screw thread ridge in a section where the recessed portion is formed is smaller than a remainder of the screw thread ridge and a ridge height in a distal end portion in a screwing direction is lower than a ridge height in a rear end portion in the screwing direction,
• the screw thread ridge in the section where the recessed portion is formed is provided with a stepped portion which overhangs sideways more than a flank surface of the remainder of the screw thread ridge and which is configured to come into contact with or approach an inner diameter edge portion of the internal screw thread when fitting the internal screw thread, and
• the recessed portion is provided on opposing flank surfaces in a same phase of screw thread ridges that are adjacent to each other, a root width between stepped portions of screw thread ridges of the sections where the recessed portion is formed which are adjacent to each other is narrower than a root width of the remainder of the screw thread ridges, and a root width between the stepped portions in the distal end portion in the screwing direction is narrower than a root width between the stepped portions in the rear end portion in the screwing direction.