A control apparatus and method, and an engine control unit for an internal combustion engine are provided for restraining a torque step and sudden fluctuations in rotation when an air/fuel mixture combustion mode is switched among a plurality of combustion modes, and for improving the fuel economy. A control apparatus of an internal combustion engine operated with a combustion mode switched between a stratified combustion mode and a uniform combustion mode comprises an ECU. The ECU calculates an ignition manipulated variable to cancel out a change in the engine rotational speed associated with the switching of the combustion mode when a first-time injection ratio changes during idle rotational speed control, and calculates an intake manipulated variable to cancel a change in the engine rotational speed caused by the ignition manipulated variable when the first-time injection ratio changes.