Abstract: The temperatures of fuel oils stored in fuel tanks 12a, 12b, 12c are separately detected by temperature sensors 52a, 52b, 52c, respectively. Based on the detection results, a CPU 49 heats a heat resistant fuel oil among the fuel oils using a heat exchanger 54, such that the temperature of an oil mixture generated by mixing the fuel oils satisfies a predetermined temperature condition. After the fuel oils including the heated heat resistant fuel oil are mixed in a blender 13, the CPU 49 detects the viscosity of the generated oil mixture using a viscometer 33. Thereafter, based on the detection result, the CPU 49 controls the mixture ratio or a heating temperature of the heat resistant fuel oil, such that the viscosity of the oil mixture satisfies a predetermined viscosity condition.