To provide a method for modifying an unused heavy carbonaceous resource or the like comprising the step of converting it to useful components including light oil and gas along with efficient desulfurization, denitrification and demetallization to effect considerable reduction of residues and raise the proportions of components with high added value in the collected product which are not found in the original resource.

This method for modifying a heavy carbonaceous resource comprises converting it to gas, naphtha, kerosene, light oil and A-heavy oil; more specifically, a conversion step: the heavy carbonaceous resource and water are homogeneously mixed together in the weight ratio of the former to the latter of 0.01-100, the mixture is reacted in the supercritical condition of water, i.e., at 480-650°C and 22-100 MPa for 30 s to 60 min to effect pyrolysis into gas, oil, water and residues, while making the desulfurization, denitrification and demetallization of the oil, a separation step: the residues are separated from the reaction product, and a fractionating step: the resulting reaction product freed from the residues is fractionated to obtain the gas, naphtha, kerosene, light oil, A-heavy oil and water.