A screw joint for a steel pipe comprising: a pin which includes a male screw portion formed by a two-step screw, and a seal surface including a tapered surface and a curved surface adjacent to the tapered surface, the seal surface of the pin being formed on an intermediate portion of the two-step screw; and a box which includes a female screw portion formed by a two-step screw, and a seal surface including a tapered surface and a curved surface adjacent to the tapered surface, the seal surface of the box being formed on an intermediate portion of the two-step screw, wherein a taper angle of the tapered surface of the seal surface of the pin is substantially the same as a taper angle of the tapered surface of the seal surface of the box, wherein the curved surface of the seal surface of the pin is formed to be adjacent to both sides of the tapered surface of the seal surface of the pin, and a length of the tapered surface of the seal surface of the box is longer than a length of the seal surface of the pin, wherein the male screw portion and the female screw portion are configured to be made-up by screwing, the pin and the box interfere with each other in a radial direction while the seal surface of the pin and the seal surface of the box contact each other in a process of the make-up, and at least a part of the seal surface of the pin comes into close contact with at least a part of the seal surface of the box over the entire circumference, wherein a contact pressure amplification mechanism, which is configured to increase a contact pressure between the seal surface of the pin and the seal surface of the box at the completion of the make-up compared to a time point during the make-up, is further provided, and wherein the contact pressure amplification mechanism is configured to increase the contact pressure between the seal surface of the pin and the seal surface of the box by: make-up between a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the male screw portion and a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the female screw portion, or a hook shoulder (24,34) which is adjacent to either a front side or a rear side of each of the seal surface of the pin and the seal surface of the box. a pin which includes a male screw portion formed by a two-step screw, and a seal surface including a tapered surface and a curved surface adjacent to the tapered surface, the seal surface of the pin being formed on an intermediate portion of the two-step screw; and a box which includes a female screw portion formed by a two-step screw, and a seal surface including a tapered surface and a curved surface adjacent to the tapered surface, the seal surface of the box being formed on an intermediate portion of the two-step screw, wherein a taper angle of the tapered surface of the seal surface of the pin is substantially the same as a taper angle of the tapered surface of the seal surface of the box, wherein the curved surface of the seal surface of the pin is formed to be adjacent to both sides of the tapered surface of the seal surface of the pin, and a length of the tapered surface of the seal surface of the box is longer than a length of the seal surface of the pin, wherein the male screw portion and the female screw portion are configured to be made-up by screwing, the pin and the box interfere with each other in a radial direction while the seal surface of the pin and the seal surface of the box contact each other in a process of the make-up, and at least a part of the seal surface of the pin comes into close contact with at least a part of the seal surface of the box over the entire circumference, wherein a contact pressure amplification mechanism, which is configured to increase a contact pressure between the seal surface of the pin and the seal surface of the box at the completion of the make-up compared to a time point during the make-up, is further provided, and wherein the contact pressure amplification mechanism is configured to increase the contact pressure between the seal surface of the pin and the seal surface of the box by: make-up between a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the male screw portion and a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the female screw portion, or a hook shoulder (24,34) which is adjacent to either a front side or a rear side of each of the seal surface of the pin and the seal surface of the box. make-up between a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the male screw portion and a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the female screw portion, or a hook shoulder (24,34) which is adjacent to either a front side or a rear side of each of the seal surface of the pin and the seal surface of the box.