PURPOSE: To provide a measuring process nearly equal to the tightening stroke of the tightening bolt in a tool shank part to be tightened by forming a measuring member by a measuring body constituted as a solid body for detecting the tightening force on the basis of the mechanical deformation.

CONSTITUTION: A fitting force measuring device is provided with a spindle tool receiving part 7 for a tool shank part, and a tightening member 11 for tightening the tool shank part to the tool receiving part 7 in the axial direction. A testing shaft part 2 fitted to the tool receiving part 7 and formed for tightenedly connecting the tightening member 11 is provided with a measuring member exerting an influence on a tightening force detecting device 3 that is influenced by the tightening force. In this case, at least one measuring member consists as a measuring body 10 for detecting the tightening force on the basis of mechanical deformation.