A synchronous-machine starting device comprising: an electric power conversion unit converting supplied electric power into AC power and supplying the AC power to an armature of a synchronous machine; an AC voltage detection unit detecting an AC voltage supplied to the armature of said synchronous machine and outputting an AC voltage signal representing said detected AC voltage; an AC current detection unit detecting an AC current supplied to the armature of said synchronous machine and outputting an AC current signal representing said detected AC current; a rotor position detection unit detecting a position of a rotor of said synchronous machine based on said AC voltage signal and said AC current signal and outputting a rotor position signal POS representing the detected position of the said rotor; and an electric power conversion control unit controlling said electric power conversion unit based on the rotor position signal POS, said rotor position detection unit including: an induction voltage operating unit calculating an induction voltage induced to the armature of said synchronous machine based on an estimated position signal representing the position of said rotor, an estimated rotational speed of said rotor, said AC voltage signal, and said AC current signal, and outputting an induction voltage signal representing said calculated induction voltage; a selection unit selecting and outputting one of said induction voltage signal received from said induction voltage operating unit and said AC voltage signal received from said AC voltage detection unit; and a feedback operating unit calculating an error of said estimated position signal based on said induction voltage signal or said AC voltage signal received from said selection unit, calculating said estimated position of the said rotor and said estimated rotational speed based on said calculated error, outputting a speed signal representing said calculated estimated rotational speed to said induction voltage operating unit, and also outputting the rotor position signal POS representing said estimated position of the said rotor to said electric power conversion control unit and said induction voltage operating unit. an electric power conversion unit converting supplied electric power into AC power and supplying the AC power to an armature of a synchronous machine; an AC voltage detection unit detecting an AC voltage supplied to the armature of said synchronous machine and outputting an AC voltage signal representing said detected AC voltage; an AC current detection unit detecting an AC current supplied to the armature of said synchronous machine and outputting an AC current signal representing said detected AC current; a rotor position detection unit detecting a position of a rotor of said synchronous machine based on said AC voltage signal and said AC current signal and outputting a rotor position signal POS representing the detected position of the said rotor; and an electric power conversion control unit controlling said electric power conversion unit based on the rotor position signal POS, said rotor position detection unit including: an induction voltage operating unit calculating an induction voltage induced to the armature of said synchronous machine based on an estimated position signal representing the position of said rotor, an estimated rotational speed of said rotor, said AC voltage signal, and said AC current signal, and outputting an induction voltage signal representing said calculated induction voltage; a selection unit selecting and outputting one of said induction voltage signal received from said induction voltage operating unit and said AC voltage signal received from said AC voltage detection unit; and a feedback operating unit calculating an error of said estimated position signal based on said induction voltage signal or said AC voltage signal received from said selection unit, calculating said estimated position of the said rotor and said estimated rotational speed based on said calculated error, outputting a speed signal representing said calculated estimated rotational speed to said induction voltage operating unit, and also outputting the rotor position signal POS representing said estimated position of the said rotor to said electric power conversion control unit and said induction voltage operating unit. an induction voltage operating unit calculating an induction voltage induced to the armature of said synchronous machine based on an estimated position signal representing the position of said rotor, an estimated rotational speed of said rotor, said AC voltage signal, and said AC current signal, and outputting an induction voltage signal representing said calculated induction voltage; a selection unit selecting and outputting one of said induction voltage signal received from said induction voltage operating unit and said AC voltage signal received from said AC voltage detection unit; and a feedback operating unit calculating an error of said estimated position signal based on said induction voltage signal or said AC voltage signal received from said selection unit, calculating said estimated position of the said rotor and said estimated rotational speed based on said calculated error, outputting a speed signal representing said calculated estimated rotational speed to said induction voltage operating unit, and also outputting the rotor position signal POS representing said estimated position of the said rotor to said electric power conversion control unit and said induction voltage operating unit.