Disclosed is a rotary actuator utilizing magnetic screws which are stably rotatable at a great torque with little torque variation. The rotary actuator comprising (a) a closed hollow cylinder, a piston sliding within the cylinder, and a rotary shaft which rotates with the sliding of the piston, has (b) a first magnetic screw member which makes a linear motion in synchronism with the sliding of the piston, and is provided with a first spiral magnetized band, and (c) a rotary shaft rotatably supported in relation to the first magnetic screw member, and having a second magnetic screw provided with a second spiral magnetized band. (d) With the sliding of the piston, the second magnetic screw receives the magnetic force from the first magnetic screw to thereby rotate the rotating shaft.