A starter generator apparatus (100, 100a, 100b) comprising: a starter generator including an armature unit including a first winding portion and a second winding portion which are arranged in parallel, each of the first winding portion and the second winding portion including a polyphase coil, and an orthogonal transformation unit having a plurality of alternate terminals connected to the first winding portion and configured to bidirectionally convert electric power between direct current and alternate current; a battery having a positive terminal connected to an input terminal of the orthogonal transformation unit, and a negative terminal connected to an output terminal of the orthogonal transformation unit; a plurality of switching elements (Q7-Q9) each interposed between one of the plurality of alternate terminals and the second winding portion and each configured to perform connection and disconnection of the second winding portion to and from the one of the plurality of alternate terminals; and a control unit configured to control the orthogonal transformation unit and the plurality of switching elements (Q7-Q9), characterized in that the starter generator includes a field unit including a permanent magnet; and in that the control unit is configured to, when the starter generator is used as a starter motor for starting an engine, select, by the plurality of switching elements (Q7-Q9), either one or both of the first winding portion and the second winding portion, based on a magnitude of a load of a piston of the engine and a coolant temperature of the engine, and when the starter generator is used as a power generator, select, by the plurality of switching elements (Q7-Q9), either one or both of the first winding portion and the second winding portion, based on a voltage of the battery. a starter generator including an armature unit including a first winding portion and a second winding portion which are arranged in parallel, each of the first winding portion and the second winding portion including a polyphase coil, and an orthogonal transformation unit having a plurality of alternate terminals connected to the first winding portion and configured to bidirectionally convert electric power between direct current and alternate current; a battery having a positive terminal connected to an input terminal of the orthogonal transformation unit, and a negative terminal connected to an output terminal of the orthogonal transformation unit; a positive terminal connected to an input terminal of the orthogonal transformation unit, and a negative terminal connected to an output terminal of the orthogonal transformation unit; a plurality of switching elements (Q7-Q9) each interposed between one of the plurality of alternate terminals and the second winding portion and each configured to perform connection and disconnection of the second winding portion to and from the one of the plurality of alternate terminals; and a control unit configured to control the orthogonal transformation unit and the plurality of switching elements (Q7-Q9), characterized in that the starter generator includes a field unit including a permanent magnet; and in that the control unit is configured to, when the starter generator is used as a starter motor for starting an engine, select, by the plurality of switching elements (Q7-Q9), either one or both of the first winding portion and the second winding portion, based on a magnitude of a load of a piston of the engine and a coolant temperature of the engine, and when the starter generator is used as a power generator, select, by the plurality of switching elements (Q7-Q9), either one or both of the first winding portion and the second winding portion, based on a voltage of the battery. when the starter generator is used as a starter motor for starting an engine, select, by the plurality of switching elements (Q7-Q9), either one or both of the first winding portion and the second winding portion, based on a magnitude of a load of a piston of the engine and a coolant temperature of the engine, and when the starter generator is used as a power generator, select, by the plurality of switching elements (Q7-Q9), either one or both of the first winding portion and the second winding portion, based on a voltage of the battery.