An electric rotating machine which comprises: a magnet rotor which has a rotor yoke having a magnetic attachment surface extending in a peripheral direction, and multiple permanent magnets for constituting a magnetic field which are intermittently arranged in the peripheral direction of the rotor yoke and are attached on the magnetic attachment surface; and a stator in which armature coils are wound around an armature core having a magnetic pole portions opposed to magnetic poles of the magnet rotor, wherein a large number of grooves which extend in the peripheral direction of the rotor yoke are provided on the magnet attachment surface of the rotor yoke.