A rotary electrical machine includes a rotational shaft supported by a housing for rotation about an axis, a rotor coupled with the rotational shaft, and a stator fixed to the housing and oriented to face the rotor. A rotor position changing mechanism moves the rotor in an axial direction of the rotational shaft using a portion of a torque transmitted between the rotational shaft and the rotor as a driving force. The rotor position changing mechanism controls the strength of the magnetic field between the rotor and the stator by moving the rotor toward or away from the stator.