A starter generator apparatus 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 a field unit including a permanent magnet; a first orthogonal transformation unit connected to the first winding portion and configured to bidirectionally convert an electric power between a direct current and an alternate current; a second orthogonal transformation unit connected to the second winding portion and configured to bidirectionally convert an electric power between a direct current and an alternate current; a battery connected in parallel with the first and second orthogonal transformation units (61, 62); a first switching element interposed between a positive terminal of the battery and an input terminal of the first orthogonal transformation unit, and configured to perform connection and disconnection between the positive terminal of the battery and the input terminal of the first orthogonal transformation unit; and a second switching element interposed between the positive terminal of the battery and an input terminal of the second orthogonal transformation unit, and configured to perform connection and disconnection between the positive terminal of the battery and the input terminal of the second orthogonal transformation unit a control unit configured to control the first and second orthogonal transformation units (61, 62) and the first and second switching elements (Q13, Q14), wherein the control unit is configured to, when the starter generator is used as a starter motor for starting an engine, select, by the switching elements (Q13, Q14), either one or both of the first wiring portion and the second wiring 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 switching elements (Q13, Q14), either one or both of the first wiring portion and the second wiring 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 a field unit including a permanent magnet; 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 a field unit including a permanent magnet; a first orthogonal transformation unit connected to the first winding portion and configured to bidirectionally convert an electric power between a direct current and an alternate current; a second orthogonal transformation unit connected to the second winding portion and configured to bidirectionally convert an electric power between a direct current and an alternate current; a battery connected in parallel with the first and second orthogonal transformation units (61, 62); a first switching element interposed between a positive terminal of the battery and an input terminal of the first orthogonal transformation unit, and configured to perform connection and disconnection between the positive terminal of the battery and the input terminal of the first orthogonal transformation unit; and a second switching element interposed between the positive terminal of the battery and an input terminal of the second orthogonal transformation unit, and configured to perform connection and disconnection between the positive terminal of the battery and the input terminal of the second orthogonal transformation unit a control unit configured to control the first and second orthogonal transformation units (61, 62) and the first and second switching elements (Q13, Q14), wherein the control unit is configured to, when the starter generator is used as a starter motor for starting an engine, select, by the switching elements (Q13, Q14), either one or both of the first wiring portion and the second wiring 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 switching elements (Q13, Q14), either one or both of the first wiring portion and the second wiring 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 switching elements (Q13, Q14), either one or both of the first wiring portion and the second wiring 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 switching elements (Q13, Q14), either one or both of the first wiring portion and the second wiring portion, based on a voltage of the battery.