For power transfer from a first DC part to a second DC part in a dual active bridge (DAB) converter by stepping up a voltage, the second bridge circuit includes a period in which a secondary winding of an insulated transformer and the second DC part conduct and a period in which ends of the secondary winding of the insulated transformer are short-circuited in the second bridge circuit. A control circuit fixes a phase difference between a first leg a the second leg, variably controls a simultaneous off period of a fifth switching element and a sixth switching element, and variably controls a simultaneous off period of a seventh switching element and an eighth switching element.