PURPOSE: To stabilize the tightening torque for a pneumatic tool having a short tightening time by installing a control means for controlling an electricity-air conversion means so that the tightening torque becomes larger than a lower limit torque when the tightening torque is less than the prescribed lower limit torque.

CONSTITUTION: When the tightening torque detected by a torque sensor exceeds a prescribed galling torque until the first timer counts the galling time, a galling judging means judges galling. Further, if the tightening torque detected by a sensor is less than a prescribed lower limit torque when the second timer counts a plurality of prescribed different judgement time from the time when a torque sensor detects a prescribed setting torque, a control means 5 controls an electricity-air conversion means 4 so that the tightening torque increases over a lower limit torque, and the air pressure supplied to a pneumatic tool 1 is adjusted. Further, if the detected tightening torque is outside a prescribed torque range when the first timer counts up, the judging means judges the tightening defect.