To accurately specify an EGR rate from an output value of an in-cylinder pressure sensor, a specifying method of an EGR rate in an internal combustion engine of the present invention acquires an output value of an in-cylinder pressure sensor at an intake stroke to calculate comparison data related to an in-cylinder pressure at the intake stroke from the acquired sensor output value. An output value of the in-cylinder pressure sensor at an exhaust stroke of the same cycle is also acquired to calculate comparison data related to the in-cylinder pressure at the exhaust stroke from the acquired sensor output value. Two comparison data are compared to specify the EGR rate of an air-fuel mixture provided for combustion from a difference between the values.