PURPOSE: To detect the fastening torque with high accuracy by storing the signal from a torque detector under no load into a sample-hold circuit and comparing directly with the maximum torque signal under fastening.

CONSTITUTION: The fastening torque of a motor driver is detected by a torque detector 2 constituted with a stress gauge irrespective of the inertia of motor M1 as an accurate torque actually loaded to a screw. The signal from said detector 2 under no load is stored into a sample-hole circuit 44 and provided together with the direct maximum signal under fastening from the torque detector 2 to a differential amplifier 45 to feed a highly accurate fastening torque detection level where drift due to the temperature variation has been cancelled is fed to, a disply 55 through a digital decision circuit 52 and AD converter. In such a manner, highly accurate detection and display of fastening torque and decision of fastening state can be performed.