PURPOSE: To prevent sticking in a fuel injection pump when it is to be changed over from heavy oil supply to light, by comparing the actual temp. with the allowable temp. difference, and by supplying the engine with fuel individually or mixed.

CONSTITUTION: A light oil at normal temp. is introduced to a switching device 100 through one of the divergent pipes 22, while a heated heavy oil through the other divergent pipe 14. A temp. sensor and a static mixer are installed at the communication part 104 of engine 13. When changing-over from heavy oil supply to light is made at said switching device 100, the temp. difference at the time of changing-over to be allowed in accordance with the type of engine is set previously, and a drive motor M is operated while the actual temp. sensed by the temp. sensor is compared with the set value of temp. difference. Thereby the fuel injection pump 7 is fed with the fuel oil with a uniform temp. to serve prevention of sticking.