PURPOSE: To provide an impact wrench, of which fastening force can be detected without enlarging the tool size.

CONSTITUTION: An output shaft 2 is provided with projections 10, 10 in both side of the rear end thereof to be formed into T-shape, and the projections 10, 10 are located so as to be fitted in nearly sector-shape recessed parts 11, 11 formed in both sides of the front surface of a hammer 32. One projection 10 is provided with a pressure detecting element 13, and the detecting signal of this pressure detecting element 13 is electrically connected to each slip ring 16a, 16b of the front end of the output shaft 2 through signal lines 14a, 14b. These slip rings 16a, 16b are electrically insulated from the output shaft 2 by a cylindrical insulator 15, and each brush 18a, 18b slides to take out the detecting signal of the pressure detecting element 13 to the outside through the brushes 18a, 18b.