A power conversion device, comprising: - a first power converter which obtains a DC voltage from a power supply; - a capacitor which is connected to an output side of the first power converter; - a second power converter which converts the DC voltage of the capacitor into an AC voltage and outputs the AC voltage to a synchronous machine; - rotation information detection means which detects rotational speed of the synchronous machine; and - control means which controls the first power converter and the second power converter, based on the rotational speed detected by the rotation information detection means, wherein - based on the rotational speed of the synchronous machine detected by the rotation information detection means, the control means controls the second power converter so that a voltage generated by the synchronous machine is lower than or equal to 1/√2 of a charge voltage of the capacitor at the time of activation of the second power converter from a coasting operation mode, and said control means controls the first power converter so that 1/√2 of the charge voltage of the capacitor is equal to or higher than the voltage generated by the synchronous machine, characterised in that: - said voltage generated by the synchronous machine is given as the voltage generated across one winding of said machine and the voltage across the capacitor is Vc - wherein the control means controls, during the coasting operation mode, the first power converter so that 1/√2 of the charge voltage of the capacitor is higher than the voltage generated by the synchronous machine. - a first power converter which obtains a DC voltage from a power supply; - a capacitor which is connected to an output side of the first power converter; - a second power converter which converts the DC voltage of the capacitor into an AC voltage and outputs the AC voltage to a synchronous machine; - rotation information detection means which detects rotational speed of the synchronous machine; and - control means which controls the first power converter and the second power converter, based on the rotational speed detected by the rotation information detection means, wherein - based on the rotational speed of the synchronous machine detected by the rotation information detection means, the control means controls the second power converter so that a voltage generated by the synchronous machine is lower than or equal to 1/√2 of a charge voltage of the capacitor at the time of activation of the second power converter from a coasting operation mode, and said control means controls the first power converter so that 1/√2 of the charge voltage of the capacitor is equal to or higher than the voltage generated by the synchronous machine, characterised in that: - said voltage generated by the synchronous machine is given as the voltage generated across one winding of said machine and the voltage across the capacitor is Vc - said voltage generated by the synchronous machine is given as the voltage generated across one winding of said machine and the voltage across the capacitor is Vc - wherein the control means controls, during the coasting operation mode, the first power converter so that 1/√2 of the charge voltage of the capacitor is higher than the voltage generated by the synchronous machine.