To provide a screw fastening machine capable of reducing a load applied to a wrist due to receiving of a reaction force produced during screw fastening, by a worker's arm.

The automatic screw fastening machine includes: a body roughly formed into an L shape in cross section and constituted of a roughly rectangular parallelpiped casing 38 along a worker's arm and a gripping part 33 connectable to the front bottom surface of the casing 38 and gripped by a worker's hand; and a reaction force receiving part 52a fixed to the rear part of the casing 38 and disposed almost parallel to the extending direction of the gripping part 33. An output shaft 35 receiving rotation of a motor 21 to rotate is disposed to partially protrude from the gripping part 33, and the casing 38 rotated around the axial line of the output shaft 35 by the reaction force produced during fastening is received by the worker's arm.