A rotating electrical machine, comprising: a stator core or a rotor core which has a plurality of semi-open slots; and a conductor wound on the stator core or the rotor core, the conductor comprising: a conductor wire bundle formed by gathering a plurality of conductor wires; and a flexible insulating covering material that covers a periphery of the conductor wire bundle, wherein the conductor has an intra-covering gap formed inward of the insulating covering material in a radial direction so that the conductor wires can move relative to each other in the intra-covering gap, and a cross-sectional shape of the insulating covering material along a perpendicular plane that is perpendicular to an extending direction in which the conductor wire bundle extends can be deformed, the semi-open slots each have a radial opening that is narrower than the width of the portion of the slot located deeper than the opening, on the inner side in the radial direction of the stator core or the rotor core, and a true circle inner diameter that is an inner diameter of the insulating covering material deformed to a true circle is larger than an opening width of the radial openings. a stator core or a rotor core which has a plurality of semi-open slots; and a conductor wound on the stator core or the rotor core, the conductor comprising: a conductor wire bundle formed by gathering a plurality of conductor wires; and a flexible insulating covering material that covers a periphery of the conductor wire bundle, wherein the conductor has an intra-covering gap formed inward of the insulating covering material in a radial direction so that the conductor wires can move relative to each other in the intra-covering gap, and a cross-sectional shape of the insulating covering material along a perpendicular plane that is perpendicular to an extending direction in which the conductor wire bundle extends can be deformed, the semi-open slots each have a radial opening that is narrower than the width of the portion of the slot located deeper than the opening, on the inner side in the radial direction of the stator core or the rotor core, and a true circle inner diameter that is an inner diameter of the insulating covering material deformed to a true circle is larger than an opening width of the radial openings. a conductor wire bundle formed by gathering a plurality of conductor wires; and a flexible insulating covering material that covers a periphery of the conductor wire bundle, wherein the conductor has an intra-covering gap formed inward of the insulating covering material in a radial direction so that the conductor wires can move relative to each other in the intra-covering gap, and a cross-sectional shape of the insulating covering material along a perpendicular plane that is perpendicular to an extending direction in which the conductor wire bundle extends can be deformed, the semi-open slots each have a radial opening that is narrower than the width of the portion of the slot located deeper than the opening, on the inner side in the radial direction of the stator core or the rotor core, and a true circle inner diameter that is an inner diameter of the insulating covering material deformed to a true circle is larger than an opening width of the radial openings.