To provide a method of utilizing yellow grease as fuel, capable of performing the disposal of the yellow grease, responding to a request for safe disposal, while safely, completely and effectively utilizing the yellow grease questionable in safety as fuel, and to provide a yellow grease mixed fuel burning boiler.

A furnace is provided with a heavy oil jet burner and a yellow grease jet burner 1 via separate pipes. Preferably, the yellow grease jet burner 1 is arranged over the heavy oil jet burner, and coagulation preventing heating means 5, 7 are installed on a yellow grease storage tank 4 and a transfer pipe 6, respectively. The heating means 5, 7 are served as heated steam pipes. In the transfer pipe 6 for the yellow grease jet burner 1, a circulation line 8 is provided for returning the yellow grease to the storage tank 4.