PURPOSE: To make the center line of round timer vertical by providing main supporting bodies, sub-supporting bodies, fitting members, nd stretching mechanisms.

CONSTITUTION: After erecting a round timer to be marked nearly vertically, a pair of vertically positioning devices 1 are respectively retained at the upper and lower sections of the timer. At this time, the square frames of the main supporting bodies 2 of the devices are penetrated by the timber and the positioning members of the devices are retained at the timber. Then the index line 7A of the upper positioning member is made to coincide with one marked horizontal line and the index line 7A of the retaining member 10 of the supporting body 2 and sub-supporting body 3 of the lower verticality deciding device is made coincident with another marked horizontal line. Thereafter, the front end of a side-face positioning member 7 of a plumb bob mechanism 4 is put on the upper marked line of the round timer by sliding and adjusting the member 7 on the timber. When the center line of the timber is vertical, the line connecting the small holes of the lower and upper vertically positioning devices 1 becomes vertical.