A threaded joint for heavy-walled oil country tubular goods, the threaded joint comprising: a pin which includes an externally-threaded member formed by applying taper thread forming to an end portion of a steel pipe having a wall thickness of 18 mm or more, a nose extending toward an end side of the steel pipe from the externally-threaded member and being a non-threaded portion, and a shoulder formed at an end of the nose; and a box which includes an internally-threaded member which is threadedly joined with the externally-threaded member, an inner periphery of a seal portion of the box which face a nose outer peripheral surface of the pin in an opposed manner, and a shoulder which is brought into contact with the shoulder of the pin; wherein, the pin and the box are joined to each other by threaded joining so that the nose outer peripheral surface and the inner periphery of the seal portion of the box are brought into a metal contact with each other in a pipe radial direction, thereby forming a seal portion which seals a fluid at the contact portions, a pin-side surface and a box-side surface which form the seal portions are formed into a convex curved surface and a tapered surface respectively, a seal thickness t s of the pin, which is a thickness of the nose in cross section in the pipe radial direction including a seal point, is set to 15 mm or less, wherein the seal point being a nose outer peripheral point at a portion where a length of an overlapping region in a pipe radial direction becomes maximum, and is determined by overlapping drawings of the same scale on which the pin and the box are described respectively such that the shoulders (12, 14) agree with each other, a shape parameter t s /D which is obtained by dividing the seal thickness t s of the pin by an outer diameter D of a pin raw pipe portion, i.e. a pin non-formed portion of the steel pipe to which pipe pin forming was applied to an end portion, is set to 0.045 or more, and a ratio of a complete thread length L of the pin, which is a length of a complete threaded portion of the externally-threaded member in a pipe axis direction, to a wall thickness t of the pin raw pipe portion, the ratio which is a complete thread length ratio L/t, is set to 4.0 or less. a pin which includes an externally-threaded member formed by applying taper thread forming to an end portion of a steel pipe having a wall thickness of 18 mm or more, a nose extending toward an end side of the steel pipe from the externally-threaded member and being a non-threaded portion, and a shoulder formed at an end of the nose; and a box which includes an internally-threaded member which is threadedly joined with the externally-threaded member, an inner periphery of a seal portion of the box which face a nose outer peripheral surface of the pin in an opposed manner, and a shoulder which is brought into contact with the shoulder of the pin; wherein, the pin and the box are joined to each other by threaded joining so that the nose outer peripheral surface and the inner periphery of the seal portion of the box are brought into a metal contact with each other in a pipe radial direction, thereby forming a seal portion which seals a fluid at the contact portions, a pin-side surface and a box-side surface which form the seal portions are formed into a convex curved surface and a tapered surface respectively, a seal thickness t s of the pin, which is a thickness of the nose in cross section in the pipe radial direction including a seal point, is set to 15 mm or less, wherein the seal point being a nose outer peripheral point at a portion where a length of an overlapping region in a pipe radial direction becomes maximum, and is determined by overlapping drawings of the same scale on which the pin and the box are described respectively such that the shoulders (12, 14) agree with each other, a shape parameter t s /D which is obtained by dividing the seal thickness t s of the pin by an outer diameter D of a pin raw pipe portion, i.e. a pin non-formed portion of the steel pipe to which pipe pin forming was applied to an end portion, is set to 0.045 or more, and a ratio of a complete thread length L of the pin, which is a length of a complete threaded portion of the externally-threaded member in a pipe axis direction, to a wall thickness t of the pin raw pipe portion, the ratio which is a complete thread length ratio L/t, is set to 4.0 or less.