Abstract: The present invention provides a method for producing a C heavy oil composition which unlikely forms sludge, has excellent ignitability and combustibility, and enable the stable operation of a combustion device such as an external combustion device, a diesel device, and a gas turbine device. The method of the present invention is comprises method for producing a C heavy oil composition with a bicyclic aromatic hydrocarbon content of 10 percent by volume or more and 45 percent by volume or less, comprising blending a cracked reformed base oil with a total aromatic content of 80 percent by volume or more and a 15° C. density of 0.90 to 1.20 g/cm3 in an amount of 1 percent by volume or more and 45 percent by volume or less on the basis of the total mass of the composition.