Abstract: An internal combustion engine in which a fuel reforming operation in a fuel reformation cylinder (2) is not executed when a gas temperature of a fuel reformation chamber (23) at a time point when a piston (22) in the fuel reformation cylinder (2) reaches a compression top dead point is estimated to fall short of a reforming operation allowable lower limit gas temperature set based on a lower limit value of a reforming reaction enabling temperature. For example, fuel is supplied from an injector (25) so that an equivalence ratio in the fuel reformation chamber (23) is less than 1. Alternatively, the fuel supply from an injector (25) is stopped. This way, a supply of non-reformed fuel from the fuel reformation cylinder (2) to an output cylinder (3) can be avoided, and knocking in the output cylinder (3) can be avoided.