For power transfer from a first DC part to a second DC part in a dual active bridge (DAB) converter by stepping down a voltage, a first bridge circuit includes a period in which the first DC part and a primary winding of an insulated transformer conduct and a period in which ends of a primary winding of the insulated transformer are short-circuited in the first bridge circuit. A second bridge circuit includes a rectification period. A control circuit variably controls 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.