Provided is a switching power supply unit being able to perform voltage conversion between two DC power supplies, and perform appropriate charge operation based on an inputted AC voltage. When a main battery is preferentially charged, an SW control section performs control such that a duty ratio is fixed in switching operation of a switching circuit, and a duty ratio is variable in switching operation of a bidirectional switching circuit. On the other hand, when an accessory battery is preferentially charged, the SW control section performs control such that a duty ratio is variable in switching operation of each of the switching circuit and the bidirectional switching circuit. When the accessory battery is preferentially charged, the SW control section may perform control such that the duty ratio is fixed in switching operation of the switching circuit, and a duty ratio is variable in switching operation of a switching element.