The present invention provides a control device for an uninterruptible power supply apparatus. When the current failure state of an AC power supply (1) is releases, an information that the AC power supply (1) is in a normal state is notarized, a synchronization control is performed for making the uninterruptible power supply apparatus (20) to generate an output voltage synchronous with the voltage of the AC power supply (1), in a state that the difference of the frequency of the AC power supply (1) be about to start and the output frequency of the uninterruptible power supply apparatus (20) is big, a synchronization control through a path of a phase difference / frequency detection circuit (21), a setting device (23), an adder(24), a limiting circuit (25), a setting device (26), a multiplier (27), an adder (28), a contact point (22b), an LPF (29), an adder (30), a V / F converter (16), and an inverter driving circuit (17). In the synchronization control of the path, the output voltage of an inverter circuit (12) and the voltage of the AC power supply (1) can be synchronized quickly without any impacts.