Based on rotation information of a synchronous machine detected by a rotation information detector, a controller causes a converter to boost a charge voltage of a capacitor so as to be higher than an induced voltage generated by the synchronous machine at a time of shifting to a coasting operation, maintains the charge voltage during the coasting operation, and performs weak field control so that the induced voltage generated by the synchronous machine becomes lower than a DC voltage at the time of shifting from the coasting operation to a power running operation or to a regenerative operation.