An EGR rate estimation method using an EGR valve opening area involves aggravated EGR rate estimation accuracy as an EGR valve is deteriorated and is unable to satisfy required accuracy when a target EGR rate is high. With a method for correcting a fuel injection amount by a fuel injection valve through estimation of a purge air-fuel ratio on the basis of variations in an air-fuel ratio variable depending on whether purging is performed, the fuel injection correction fails to accommodate the change in the air-fuel ratio when concentration of fuel evaporative emissions adsorbed by an activated carbon of a canister is high, resulting in reduced conversion efficiency of a catalyst. An arrangement includes an introduction port that is disposed in an intake pipe and through which a gas other than fresh air flows in the intake pipe and humidity sensors disposed upstream and downstream, respectively, of the introduction port. The EGR rate or the purge air-fuel ratio in the intake pipe is estimated using detection values of the respective humidity sensors.