A control device to be applied to an internal combustion engine including a fuel reformation device capable of generating reformed fuel through a fuel reforming operation and an output cylinder to which the reformed fuel generated in the fuel reformation device is supplied, the output cylinder configured to yield an engine power by combusting the reformed fuel, the control device comprising: a reforming operation control unit configured not to execute the fuel reforming operation in the fuel reforming device, when a total heat generation quantity on assumption that reformed fuel generated within a predetermined period in the fuel reformation device is combusted is estimated as to be more than a required reformed-fuel heat generation quantity required in the predetermined period for the output cylinder, characterized in that the fuel reformation cylinder and the output cylinder are both structured as a reciprocation type, and in that the total heat generation quantity on assumption that the reformed fuel generated within the predetermined period in the fuel reformation device is combusted is calculated based on at least an amount of gas introduced to the fuel reformation device and a quantity of fuel supplied to the fuel reformation device and the temperature and the pressure of gas from the fuel reformation device, based on the output signal from a taken-in gas pressure sensor corresponding to the pressure of the taken-in gas flowing in a fuel reformation cylinder air-intake passage and on the output signal from a taken-in gas temperature sensor corresponding to the temperature of the taken-in gas flowing in the fuel reformation cylinder air-intake passage or the total heat generation quantity is calculated by measuring the gas temperature and the gas pressure at an outlet of the fuel reformation chamber. a reforming operation control unit configured not to execute the fuel reforming operation in the fuel reforming device, when a total heat generation quantity on assumption that reformed fuel generated within a predetermined period in the fuel reformation device is combusted is estimated as to be more than a required reformed-fuel heat generation quantity required in the predetermined period for the output cylinder, characterized in that the fuel reformation cylinder and the output cylinder are both structured as a reciprocation type, and in that the total heat generation quantity on assumption that the reformed fuel generated within the predetermined period in the fuel reformation device is combusted is calculated based on at least an amount of gas introduced to the fuel reformation device and a quantity of fuel supplied to the fuel reformation device and the temperature and the pressure of gas from the fuel reformation device, based on the output signal from a taken-in gas pressure sensor corresponding to the pressure of the taken-in gas flowing in a fuel reformation cylinder air-intake passage and on the output signal from a taken-in gas temperature sensor corresponding to the temperature of the taken-in gas flowing in the fuel reformation cylinder air-intake passage or the total heat generation quantity is calculated by measuring the gas temperature and the gas pressure at an outlet of the fuel reformation chamber.