PURPOSE: To prevent the forgetting to tighten screws by automatically counting the quantity of tightened screws from the output of a pressure sensor, comparing the quantity with a set quantity and outputting the result, when a product is assembled by carrying out screw tightening work by using an air screw driver.

CONSTITUTION: An air screw driver is connected with a hose connected with an air compressor, and a pressure sensor is interposed midway in the hose. The air pressure which changes in tightening a screw by using the air driver is detected by the pressure sensor, and the number of times of tightening the screw is counted in a main circuit on the basis of the detection signal, and the number which is set by a quantity setting device and is displayed on a display device is reduced one by one. When the number displayed on the display device becomes 0, the completion of the screw tightening for the set quantity is informed through a monitor LED. At the same time, the set number of quantity is preset on the display device, and a buzzer is operated.