A control device for an internal combustion engine that includes a first actuator (2, 8; 2, 8,10) that changes an amount of air that is drawn into a cylinder, a second actuator that supplies fuel into a cylinder, and a third actuator that ignites an air-fuel mixture in a cylinder, and that is configured to be capable of selecting between operation according to a first air-fuel ratio and operation according to a second air-fuel ratio that is leaner than the first air-fuel ratio, the control device comprising: requested torque reception means for receiving a requested torque; reference value calculation means for calculating a first reference value corresponding to a torque based on an engine speed of the internal combustion engine, and calculating as a second reference value a value of the torque that will be obtained if an air-fuel ratio is adjusted to the first air-fuel ratio at an air amount with which the first reference value can be achieved under the second air-fuel ratio; target air-fuel ratio switching means for setting a target air-fuel ratio to the first air-fuel ratio in response to the requested torque being greater than the first reference value, and switching the target air-fuel ratio from the first air-fuel ratio to the second air-fuel ratio in response to a decrease in the requested torque to an amount that is equal to or less than the first reference value; target air amount calculation means (162; 182) for back-calculating from the requested torque a target air amount for achieving the requested torque by using a parameter that provides a conversion efficiency of an air amount to torque; parameter value changing means (404; 406; 408) for starting to change a value of the parameter in a direction that lowers the conversion efficiency in response to a decrease in the requested torque to a value that is equal to or less than the second reference value, and gradually changing the value of the parameter in a direction that lowers the conversion efficiency as the requested torque further decreases from the second reference value to the first reference value; first actuator control means (164, 166, 178; 166, 178, 184, 186) for determining an operation amount of the first actuator (2, 8; 2, 8,10) based on the target air amount, and operating the first actuator (2, 8; 2, 8,10) in accordance with the operation amount; second actuator control means (174, 176) for determining a fuel supply amount based on the target air-fuel ratio, and operating the second actuator in accordance with the fuel supply amount; and third actuator control means (168, 170, 172) for determining an ignition timing for achieving the requested torque based on the requested torque and a torque that is estimated based on the operation amount of the first actuator (2, 8; 2, 8,10) and the target air-fuel ratio, and operating the third actuator in accordance with the ignition timing. requested torque reception means for receiving a requested torque; reference value calculation means for calculating a first reference value corresponding to a torque based on an engine speed of the internal combustion engine, and calculating as a second reference value a value of the torque that will be obtained if an air-fuel ratio is adjusted to the first air-fuel ratio at an air amount with which the first reference value can be achieved under the second air-fuel ratio; target air-fuel ratio switching means for setting a target air-fuel ratio to the first air-fuel ratio in response to the requested torque being greater than the first reference value, and switching the target air-fuel ratio from the first air-fuel ratio to the second air-fuel ratio in response to a decrease in the requested torque to an amount that is equal to or less than the first reference value; target air amount calculation means (162; 182) for back-calculating from the requested torque a target air amount for achieving the requested torque by using a parameter that provides a conversion efficiency of an air amount to torque; parameter value changing means (404; 406; 408) for starting to change a value of the parameter in a direction that lowers the conversion efficiency in response to a decrease in the requested torque to a value that is equal to or less than the second reference value, and gradually changing the value of the parameter in a direction that lowers the conversion efficiency as the requested torque further decreases from the second reference value to the first reference value; first actuator control means (164, 166, 178; 166, 178, 184, 186) for determining an operation amount of the first actuator (2, 8; 2, 8,10) based on the target air amount, and operating the first actuator (2, 8; 2, 8,10) in accordance with the operation amount; second actuator control means (174, 176) for determining a fuel supply amount based on the target air-fuel ratio, and operating the second actuator in accordance with the fuel supply amount; and third actuator control means (168, 170, 172) for determining an ignition timing for achieving the requested torque based on the requested torque and a torque that is estimated based on the operation amount of the first actuator (2, 8; 2, 8,10) and the target air-fuel ratio, and operating the third actuator in accordance with the ignition timing.