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;