A control device includes a cylinder pressure sensor, a driving condition detector, a reference crank angle setter, a reference cylinder pressure calculator, an EGR ratio estimator, and a controller. The cylinder pressure sensor detects a cylinder pressure inside a cylinder. The driving condition detector detects a driving condition in an internal combustion engine. The reference crank angle setter calculates, according to the driving condition, a reference crank angle immediately before which mixture gas starts combusting. The reference cylinder pressure calculator calculates a reference cylinder pressure at the reference crank angle based on temperature characteristics of a heat capacity ratio of the mixture gas under a condition. The EGR ratio estimator calculates an EGR ratio based on a pressure difference between the reference cylinder pressure and the cylinder pressure at the reference crank angle. The controller controls the internal combustion engine according to the EGR ratio.