To achieve easy measurement of clamping force of a machine tool.

While slidably securing a shank 1 in the axial direction within a device body 9, a contact type switch 3 detecting a movement of the shank 1 is provided. While a belleville spring 15 is fitted to the outer peripheral surface of a small diameter shaft 12 of the shank 1 and arranged between an inner wall 23a of the device body 9 to urge the shank 1 backward with the belleville spring 15. The elastic force of the belleville spring 15 is made equivalent to the clamping force of the clamping mechanism of the machine tool. In the switch 3, the top end of a conductor lever 41 is axially supported by a switch retaining member 4, wherein the lever 41 is urged clockwise with a compression coil spring 43. While providing one contact 44 to the bottom end of the lever 41, the other contact 45 is provided to the device body 9 so as to construct a circuit which passes a switch 3, a battery 5 and an indicator lamp 7. Sufficient retraction of the clamping mechanism of the shank 1 leads to a mutual contact between the contacts 44 and 45 and to lighting of an indicator lamp 7 to indicate sufficiency of the clamping force.