A screw joint for a pipe comprising: a pin member having a male threaded portion, a nose portion extending toward a pipe end side from the male threaded portion, and a shoulder portion formed on a distal end of the nose portion; and a box member having a female threaded portion which is threadedly joined to the male threaded portion by thread engagement, an inner peripheral surface which faces an outer peripheral surface of the nose portion of the pin member in an opposed manner, and a shoulder portion which is brought into contact with the shoulder portion of the pin member, wherein the outer peripheral surface of the nose portion of the pin member and the inner peripheral surface of the box member are brought into a metal to metal contact due to joining of the pin member and the box member by threaded engagement, and a contact interface forms a seal surface, the outer peripheral surface of the nose portion of the pin member is formed into an outwardly projecting convex curve as viewed in cross section taken along an axial direction of the pin member, the convex curve is formed into a curved shape where a composite R curve is formed by sequentially connecting a plurality of outwardly convex arcs having different radii of curvature R from each other on a generating line of a cylindrical portion disposed adjacent to the male threaded portion, a radius of curvature R of the arc is increased as the arc is away from the male threaded portion, and a tangent of the arc on a connecting point agrees with a tangent of the arc which is a connection counterpart, the inner peripheral surface of the box member is formed into a tapered surface which interferes with the outer peripheral surface of the nose portion of the pin member at the time of joining with the pin member, and wherein an outer diameter of a raw pipe portion of the pin member is set to 168.3mm or less, and a length of the nose portion is set to 5mm or more and less than 20mm. a pin member having a male threaded portion, a nose portion extending toward a pipe end side from the male threaded portion, and a shoulder portion formed on a distal end of the nose portion; and a box member having a female threaded portion which is threadedly joined to the male threaded portion by thread engagement, an inner peripheral surface which faces an outer peripheral surface of the nose portion of the pin member in an opposed manner, and a shoulder portion which is brought into contact with the shoulder portion of the pin member, wherein the outer peripheral surface of the nose portion of the pin member and the inner peripheral surface of the box member are brought into a metal to metal contact due to joining of the pin member and the box member by threaded engagement, and a contact interface forms a seal surface, the outer peripheral surface of the nose portion of the pin member is formed into an outwardly projecting convex curve as viewed in cross section taken along an axial direction of the pin member, the convex curve is formed into a curved shape where a composite R curve is formed by sequentially connecting a plurality of outwardly convex arcs having different radii of curvature R from each other on a generating line of a cylindrical portion disposed adjacent to the male threaded portion, a radius of curvature R of the arc is increased as the arc is away from the male threaded portion, and a tangent of the arc on a connecting point agrees with a tangent of the arc which is a connection counterpart, the inner peripheral surface of the box member is formed into a tapered surface which interferes with the outer peripheral surface of the nose portion of the pin member at the time of joining with the pin member, and wherein an outer diameter of a raw pipe portion of the pin member is set to 168.3mm or less, and a length of the nose portion is set to 5mm or more and less than 20mm.