To increase productivity by shortening the cycle time of melting and tapping work for a flotation melting device.

A changeover device 9 is provided for changing over an electrical path coming from an alternating-current power supply 3 from one flotation melting device to another, melting preparations for one flotation melting device are made during the melting and tapping period of another flotation melting device connected to the alternating-current power supply 3, and after the tapping of this flotation melting device has been completed, the electrical path is switched to that flotation melting device, so that it can perform melting, thus the intervals of melting/tapping work are shortened, whereby productivity is increased.