An apparatus for estimating a temperature of an exhaust gas which is exhausted from an internal combustion engine to an exhaust manifold. The apparatus estimates the temperature of the exhaust gas in the exhaust manifold, based on a temperature increment caused by an adiabatic compression until a gas introduced in an intake manifold is exhausted to the exhaust manifold, and a temperature increment obtained by dividing the amount of heat lost in the exhaust gas among the amount of heat introduced into cylinders by the diluted gas flow of the exhaust gas in the exhaust manifold.