A synchronous machine starting device comprising: a power conversion unit for converting supplied power into AC power and supplying the AC power to an armature of a synchronous machine; an electric power line for supplying said AC power from said power conversion unit to said synchronous machine; an AC voltage detection unit (8, 58) for detecting AC voltage supplied to the armature of said synchronous machine through said electric power line; a rotor position detection unit for detecting a rotor position of said synchronous machine, based on the detected AC voltage; a power conversion control unit for controlling said power conversion unit, based on the detected rotor position, characterized in that said AC voltage detection unit (8, 58) has a first output end and a second output end isolated from said electric power line, transforms AC voltage supplied through said electric power line at a first ratio to output the transformed voltage from said first output end, and transforms AC voltage supplied through said electric power line at a second ratio and then limits the transformed voltage to a prescribed positive voltage value or lower and a prescribed negative voltage value or higher for output from said second output end; and a detected voltage selection unit for selecting the voltage received from said first output end when the rotational speed of the rotor of the synchronous machine is a prescribed value or higher or when the magnitude of the terminal voltage of the synchronous machine becomes a prescribed value or greater because of acceleration of synchronous machine and the voltage received from said second output end when the rotational speed of the rotor of the synchronous machine is lower than a prescribed value or when the magnitude of the terminal voltage of the synchronous machine is smaller than a prescribed value, and outputting the selected one to said rotor position detection unit. a power conversion unit for converting supplied power into AC power and supplying the AC power to an armature of a synchronous machine; an electric power line for supplying said AC power from said power conversion unit to said synchronous machine; an AC voltage detection unit (8, 58) for detecting AC voltage supplied to the armature of said synchronous machine through said electric power line; a rotor position detection unit for detecting a rotor position of said synchronous machine, based on the detected AC voltage; a power conversion control unit for controlling said power conversion unit, based on the detected rotor position, characterized in that said AC voltage detection unit (8, 58) has a first output end and a second output end isolated from said electric power line, transforms AC voltage supplied through said electric power line at a first ratio to output the transformed voltage from said first output end, and transforms AC voltage supplied through said electric power line at a second ratio and then limits the transformed voltage to a prescribed positive voltage value or lower and a prescribed negative voltage value or higher for output from said second output end; and a detected voltage selection unit for selecting the voltage received from said first output end when the rotational speed of the rotor of the synchronous machine is a prescribed value or higher or when the magnitude of the terminal voltage of the synchronous machine becomes a prescribed value or greater because of acceleration of synchronous machine and the voltage received from said second output end when the rotational speed of the rotor of the synchronous machine is lower than a prescribed value or when the magnitude of the terminal voltage of the synchronous machine is smaller than a prescribed value, and outputting the selected one to said rotor position detection unit.