PURPOSE: To provide a jig for calibration which can be applied to plural kinds of nuts by providing a shaft part fitted with nut fitting recess parts corresponding to plural nut diameters in a jig for calibration which can be applied to an axial force meter for testing the axial force of a bolt in a building site and is used for loosening a nut after testing.

CONSTITUTION: A measuring device (axial force meter) 1 for measuring the axial force of a bolt comprises a bolt bushing where the head of a bolt is inserted to prevent following turn of a bolt at the time of fastening, and a disc member 4 replaced according to the size of a bolt. The oil pressure generated by a load device when a nut is fastened to the disc member 4 is converted to the axial force and measured by a meter 2. In this case, at the time of remiving a bolt and a nut, a jig 10 for calibration is used. The jig comprises a shaft part 12 fitted with two-state nut fitting recessed parts 11 and a casing member 13 which is installed on the outer periphery of the part and fitted with a projection 14 removably fitted in a hole of the disc member 4, wherein a nut is loosened by a driving power source such as an electric wrench connected to the shaft part 12.