A stator, in which a rotor is rotatably accommodated, is provided with a cylindrical yoke and a magnet that is arranged so as to extend along an inner circumference of the yoke, and has a plurality of magnetic poles generating a magnetic field operable to rotate the rotor. A tip tool is coupled to the rotor and operable to perform a prescribed working in accordance with the rotation of the rotor. The yoke is includes a plurality of steel plates laminated together to form a cylindrical yoke. At least one of a groove and a rib is provided on at least one of the inner circumference of the yoke and an outer circumferential of the magnet so as to extend in an axial direction of the cylindrical yoke. The at least one of the groove and the rib is provided at a boundary between the magnetic poles.