The air motor of the present invention has an air chamber for taking compressed air in a rotor body fitted to a rotary shaft, forms a jet hole opening in the tangential direction of the rotor on the periphery of the rotor, jets compressed air from the jet hole, and gives torque to the rotor by the reaction of the jetted air. An air reservoir is installed between the air chamber and the jet hole which are connected to the air reservoir. When the load of the air motor increases, the speed of the rotor instantaneously decreases. However, the decreased speed immediately increases because a lot of compressed air stored in the air reservoir continuously jets from the jet hole.