A method for manufacturing a rotor core, comprising: conveying a core body to a magnet mounting device, mounting, in the magnet mounting device, a plurality of permanent magnets into a plurality of magnet insertion holes of the core body, wherein the magnet insertion holes are passing through the core body in a height direction, along the central axis (Ax), wherein the magnet mounting device includes a resin charging mechanism and a detection mechanism, wherein the resin charging mechanism includes a lower die and a holding member, wherein the lower die includes a plurality of first projecting portions, wherein the detection mechanism includes a height detecting unit, wherein mounting the magnets into the magnet insertion holes includes the following steps: detecting, by the height detecting unit, heights of the first projecting portions formed on the lower die; placing the core body onto the lower die such that the first projecting portions are positioned in the magnet insertion holes when the height detecting unit has determined that the heights of the first projecting portions are within a set range; bringing the permanent magnets in the magnet insertion holes into contact with upper ends, in the height direction of the first projecting portions; placing the holding member onto the core body after bringing the permanent magnets into contact with the upper ends of the first projecting portions, wherein the holding member is configured to hold the core body with the lower die and to sandwich the core body with the lower die in the height direction, after placing the holding member onto the core body, injecting melted resin into the magnet insertion holes in which the permanent magnets have been inserted for producing the rotor core; and after injecting melted resin into the magnet insertion holes, removing the lower die and the holding member from the rotor core and conveying the lower die to the height detecting unit in order to produce another rotor core. conveying a core body to a magnet mounting device, mounting, in the magnet mounting device, a plurality of permanent magnets into a plurality of magnet insertion holes of the core body, wherein the magnet insertion holes are passing through the core body in a height direction, along the central axis (Ax), wherein the magnet mounting device includes a resin charging mechanism and a detection mechanism, wherein the resin charging mechanism includes a lower die and a holding member, wherein the lower die includes a plurality of first projecting portions, wherein the detection mechanism includes a height detecting unit, wherein mounting the magnets into the magnet insertion holes includes the following steps: detecting, by the height detecting unit, heights of the first projecting portions formed on the lower die; placing the core body onto the lower die such that the first projecting portions are positioned in the magnet insertion holes when the height detecting unit has determined that the heights of the first projecting portions are within a set range; bringing the permanent magnets in the magnet insertion holes into contact with upper ends, in the height direction of the first projecting portions; placing the holding member onto the core body after bringing the permanent magnets into contact with the upper ends of the first projecting portions, wherein the holding member is configured to hold the core body with the lower die and to sandwich the core body with the lower die in the height direction, after placing the holding member onto the core body, injecting melted resin into the magnet insertion holes in which the permanent magnets have been inserted for producing the rotor core; and after injecting melted resin into the magnet insertion holes, removing the lower die and the holding member from the rotor core and conveying the lower die to the height detecting unit in order to produce another rotor core. detecting, by the height detecting unit, heights of the first projecting portions formed on the lower die; placing the core body onto the lower die such that the first projecting portions are positioned in the magnet insertion holes when the height detecting unit has determined that the heights of the first projecting portions are within a set range; bringing the permanent magnets in the magnet insertion holes into contact with upper ends, in the height direction of the first projecting portions; placing the holding member onto the core body after bringing the permanent magnets into contact with the upper ends of the first projecting portions, wherein the holding member is configured to hold the core body with the lower die and to sandwich the core body with the lower die in the height direction, after placing the holding member onto the core body, injecting melted resin into the magnet insertion holes in which the permanent magnets have been inserted for producing the rotor core; and after injecting melted resin into the magnet insertion holes, removing the lower die and the holding member from the rotor core and conveying the lower die to the height detecting unit in order to produce another rotor core.