A screw joint for an oil well pipe, comprising: a pin that is provided with a male screw portion which is a male taper screw, at one end of a steel pipe; and a box that is provided with a female screw portion which is a female taper screw fitted to the male screw portion, at one end of the steel pipe, wherein a structure, in which the pin and the box are in metal contact with each other to seal a fluid, is provided at least at one place of a seal portion on an outer peripheral surface side of a pipe end side of the pin and a seal portion on an inner peripheral surface side of a pipe end side of the box, characterized in that the screw joint is an integral type screw joint, wherein a minimum value L min (mm) of a screw length L (mm) in screw rows of the male taper screw and the female taper screw is defined by expression, and wherein the screw length L (mm) in the screw rows satisfies expression. L min = t × D − t / α t × D t / √ 3 × joint efficiency L min × 1.0 ≤ L ≤ L min × 2.5 Here, t: a pipe thickness (mm) of each of raw pipe portions which are unprocessed portions (6, 7) of the box and the pin, D: a pipe diameter (mm) of each of the raw pipe portions which are unprocessed portions (6, 7) of the box and the pin, αt: a ratio of an effective screw length to the screw length L, a shorter effective screw length of the box or the pin being chosen for the effective screw length, D t: an average screw diameter (mm) of screw valleys in the taper screw having a shorter effective screw length of the box or the pin, and joint efficiency: tensile strength of the joint portion/tensile strength of the raw pipe portion. a pin that is provided with a male screw portion which is a male taper screw, at one end of a steel pipe; and a box that is provided with a female screw portion which is a female taper screw fitted to the male screw portion, at one end of the steel pipe, wherein a structure, in which the pin and the box are in metal contact with each other to seal a fluid, is provided at least at one place of a seal portion on an outer peripheral surface side of a pipe end side of the pin and a seal portion on an inner peripheral surface side of a pipe end side of the box, characterized in that the screw joint is an integral type screw joint, wherein a minimum value L min (mm) of a screw length L (mm) in screw rows of the male taper screw and the female taper screw is defined by expression, and wherein the screw length L (mm) in the screw rows satisfies expression. L min = t × D − t / α t × D t / √ 3 × joint efficiency L min × 1.0 ≤ L ≤ L min × 2.5 Here, t: a pipe thickness (mm) of each of raw pipe portions which are unprocessed portions (6, 7) of the box and the pin, D: a pipe diameter (mm) of each of the raw pipe portions which are unprocessed portions (6, 7) of the box and the pin, αt: a ratio of an effective screw length to the screw length L, a shorter effective screw length of the box or the pin being chosen for the effective screw length, D t: an average screw diameter (mm) of screw valleys in the taper screw having a shorter effective screw length of the box or the pin, and joint efficiency: tensile strength of the joint portion/tensile strength of the raw pipe portion. Here, t: a pipe thickness (mm) of each of raw pipe portions which are unprocessed portions (6, 7) of the box and the pin, D: a pipe diameter (mm) of each of the raw pipe portions which are unprocessed portions (6, 7) of the box and the pin, αt: a ratio of an effective screw length to the screw length L, a shorter effective screw length of the box or the pin being chosen for the effective screw length, D t: an average screw diameter (mm) of screw valleys in the taper screw having a shorter effective screw length of the box or the pin, and joint efficiency: tensile strength of the joint portion/tensile strength of the raw pipe portion.