PURPOSE: To make it possible to easily and surely mount a fixed jig of a positioning leveling string without specially welding it and to easily correct its mounting position.

CONSTITUTION: A fixed jig is constituted of a U-shaped hardware 1 consisting of a first side wall 1a, a second side wall 1b and a third side wall 1c, a bolt 3 capable of being inserted in a bolt hole 2 bored in the first side wall 1a and a bar material 6 capable of being mounted to a nut 5 provided outside the second side wall 1b and third side wall 1c in a detachable manner. A flange of a skeleton beam is held in a space between the front end of the bolt 3 inserted in the bolt hole 2 of the first side wall 1a and second side wall 1b.