A control device to be applied to an internal combustion engine having a fuel reformation cylinder and an output cylinder, wherein the compression end gas temperature of the fuel reformation cylinder is adjusted by changing an EGR gas amount, an effective compression ratio, or a polytropic number according to an equivalence ratio in the fuel reformation cylinder so as to achieve a target concentration of a specific gas component in reformed fuel generated in the fuel reformation cylinder. The target concentration is defined according to a required engine power.