A resonant converter which can realize its easy design, eliminate the need for its adjustment, realize reliable soft switching operation, and increase an efficiency. A driver DRV of a main switching device uses a PLL circuit PLL to apply frequency tracking control in such a manner that a driving frequency fsw is made to track a resonant frequency fr (or 1/N thereof, N being an odd number) and that the phase of the driving frequency fsw leads the current zero phase of the resonant frequency fr always by a desired time. As a result, such an optimum condition can be kept that the main switching device is triggered immediately before the zero-cross point of a resonant current. Thus the resonant converter which can always satisfy the optimum operational conditions, realize soft switching operation, and increase an efficiency can be realized with an easy design and a manufacturing adjustment-free arrangement.