A friction-stir welding tool, comprising: a front side shoulder (6;26;36) extending in the direction of an axis; a rear side shoulder (5;25;35) which is provided to oppose the front side shoulder (6;26;36) in the direction of the axis, and is arranged to come into contact with plate materials to be welded to interpose an abutment portion (W1a) of the plate materials between the front side shoulder (6;26;36) and the rear side shoulder (5;25;35); and a probe (7;27;37) which is disposed to penetrate the plate materials between the front side shoulder (6;26;36) and the rear side shoulder (5;25;35), extends in the direction of the axis, and connects the front side shoulder (6;26;36) and the rear side shoulder (5;25;35) to each other, wherein the probe (7;27;37) includes a first screw part provided on at least one end section in the direction of the axis, and a stirring groove which is provided in a portion of the probe (7;27;37) in which the first screw part is not provided, and wherein at least one of the front side shoulder (6;26;36) and the rear side shoulder (5;25;35) includes a second screw part which is a female screw provided in a hole part formed in the direction of the axis moving away from an end surface (5a;25a;35a;6a;26a;36a) of the at least one of the front side shoulder (6;26;36) and the rear side shoulder (5;25;35), and the second screw part is in threaded engagement with the first screw part so that, when the state of the threaded engagement is changed in that the front side shoulder (6;26;36) and the rear side shoulder (5;25;35) are rotated relative to each other, the front side shoulder (6;26;36) and the rear side shoulder (5;25;35) axially move closer to each other, wherein lead angles of the first screw part and the second screw part are set such that, in a state where a screwing force generated by a friction force between the second screw part and the first screw part and a reaction force of a welding pressure of the rear side shoulder (5;25;35) received from the plate materials are balanced with each other, the welding pressure of the rear side shoulder (5;25;35) with respect to the plate materials is suitable for the welding, and characterised in that a diameter-expanding hole part having a larger inner diameter than an outer diameter of the probe (7;27;37) is formed between the second screw part and the end surface (5;25;35a;6a;26a;36a) in the hole part. a front side shoulder (6;26;36) extending in the direction of an axis; a rear side shoulder (5;25;35) which is provided to oppose the front side shoulder (6;26;36) in the direction of the axis, and is arranged to come into contact with plate materials to be welded to interpose an abutment portion (W1a) of the plate materials between the front side shoulder (6;26;36) and the rear side shoulder (5;25;35); and a probe (7;27;37) which is disposed to penetrate the plate materials between the front side shoulder (6;26;36) and the rear side shoulder (5;25;35), extends in the direction of the axis, and connects the front side shoulder (6;26;36) and the rear side shoulder (5;25;35) to each other, wherein the probe (7;27;37) includes a first screw part provided on at least one end section in the direction of the axis, and a stirring groove which is provided in a portion of the probe (7;27;37) in which the first screw part is not provided, and wherein at least one of the front side shoulder (6;26;36) and the rear side shoulder (5;25;35) includes a second screw part which is a female screw provided in a hole part formed in the direction of the axis moving away from an end surface (5a;25a;35a;6a;26a;36a) of the at least one of the front side shoulder (6;26;36) and the rear side shoulder (5;25;35), and the second screw part is in threaded engagement with the first screw part so that, when the state of the threaded engagement is changed in that the front side shoulder (6;26;36) and the rear side shoulder (5;25;35) are rotated relative to each other, the front side shoulder (6;26;36) and the rear side shoulder (5;25;35) axially move closer to each other, wherein lead angles of the first screw part and the second screw part are set such that, in a state where a screwing force generated by a friction force between the second screw part and the first screw part and a reaction force of a welding pressure of the rear side shoulder (5;25;35) received from the plate materials are balanced with each other, the welding pressure of the rear side shoulder (5;25;35) with respect to the plate materials is suitable for the welding, and characterised in that a diameter-expanding hole part having a larger inner diameter than an outer diameter of the probe (7;27;37) is formed between the second screw part and the end surface (5;25;35a;6a;26a;36a) in the hole part.