PURPOSE: To prevent damage of a threaded groove by performing at least one cycle of screwing and unscrewing component parts of a screwthreaded joint for the assembly of tubes.

CONSTITUTION: Spanners 1, 2 each obtained by rotatably arranging a hollow journal 3, 4 in a bearing forming hole 5, 6 of a spanner support structure 7, 8 has an arm 9, 10 provided therefor. The arms 9, 10 each have an end 11, 12 to which an end of each driving jack 13, 14 is pivotally interconnected. Next, the other end of each jack 13, 14 is interconnected to each portion 17, 18 which is at a lower portion of each spanner support structure 7, 8, so as to be pivotal about a connecting shaft. The spanner support structure 7, 8 is mounted on a common frame body 19 via each elastic support portion 20, 21. The spanners 1, 2 are arranged in juxtaposition at a small interval which is changed during screwing or unscrewing of two male and female component parts of a screwthreaded joint. Further, necessary axial play is provided such that the hollow shaft 3, 4 can be moved in the axial direction by a corresponding amount in the hole 5, 6.