The LLC resonant converter includes a bridge circuit configured to receive a DC input voltage, an LLC resonant circuit connected to the bridge circuit, a transformer connected to the LLC resonant circuit, a rectifier circuit connected to the transformer and configured to send out a converted DC voltage, a resonant capacitor changeover circuit, a bridge circuit control section, and a resonant capacitor changeover control section. When the input voltage exceeds a changeover voltage, the operating frequency is raised higher than the resonance frequency, and thereafter the switch is turned off.