A rotor , comprising: a rotor core ; and a plurality of magnetic pole parts provided to the rotor core , wherein the plurality of magnetic pole parts are lined up along an outer circumferential surface of the rotor core , wherein the rotor is combined with a stator disposed radially outward of the rotor core , and a magnetic force of the plurality of magnetic pole parts is changeable by a given magnetic flux generated by the stator , and wherein each magnetic pole part of the plurality of magnetic pole parts includes: a fixed magnetic-force magnet disposed so that a magnetizing direction thereof is oriented in a radial direction of the rotor core : first variable magnetic-force magnets disposed at positions radially outward of the fixed magnetic-force magnet on both sides of the fixed magnetic-force magnet in a circumferential direction of the rotor core so that magnetizing directions thereof are oriented in the circumferential direction, a magnetization state of the first variable magnetic-force magnets being changeable by the given magnetic flux; a first cavity part formed in the rotor core so as to extend between a position radially inward of the fixed magnetic-force magnet and a position radially inward of the first variable magnetic-force magnets ; characterized in that each magnetic pole part of the plurality of magnetic pole parts additionally includes: a second cavity part formed in the rotor core so as to be separated radially outward from the first cavity part , the second cavity part extending toward the first variable magnetic-force magnet in the circumferential direction from a position radially outward of a surface on a first variable magnetic-force magnet side of the fixed magnetic-force magnet . a rotor core ; and a plurality of magnetic pole parts provided to the rotor core , wherein the plurality of magnetic pole parts are lined up along an outer circumferential surface of the rotor core , wherein the rotor is combined with a stator disposed radially outward of the rotor core , and a magnetic force of the plurality of magnetic pole parts is changeable by a given magnetic flux generated by the stator , and wherein each magnetic pole part of the plurality of magnetic pole parts includes: a fixed magnetic-force magnet disposed so that a magnetizing direction thereof is oriented in a radial direction of the rotor core : first variable magnetic-force magnets disposed at positions radially outward of the fixed magnetic-force magnet on both sides of the fixed magnetic-force magnet in a circumferential direction of the rotor core so that magnetizing directions thereof are oriented in the circumferential direction, a magnetization state of the first variable magnetic-force magnets being changeable by the given magnetic flux; a first cavity part formed in the rotor core so as to extend between a position radially inward of the fixed magnetic-force magnet and a position radially inward of the first variable magnetic-force magnets ; first variable magnetic-force magnets disposed at positions radially outward of the fixed magnetic-force magnet on both sides of the fixed magnetic-force magnet in a circumferential direction of the rotor core so that magnetizing directions thereof are oriented in the circumferential direction, a magnetization state of the first variable magnetic-force magnets being changeable by the given magnetic flux; a first cavity part formed in the rotor core so as to extend between a position radially inward of the fixed magnetic-force magnet and a position radially inward of the first variable magnetic-force magnets ; <EOS>