An apparatus for estimating an exhaust gas temperature in an exhaust manifold of an internal combustion engine, the apparatus comprising: an estimating means for estimating the exhaust gas temperature (Tem), based on a temperature increment (Tim') caused by an adiabatic compression until a gas introduced into an intake manifold of the internal combustion engine is exhausted to the exhaust manifold, and a temperature increment (ΔTf) caused by an amount of heat obtained by dividing an amount of heat (Qexf) lost in an exhaust gas among an amount of heat (Qf) introduced into cylinders (2a-f) of the internal combustion engine by a diluted gas flow (Gexh) of the exhaust gas in the exhaust manifold; an intake gas temperature acquiring means for acquiring an intake gas temperature (Tim) in the intake manifold; an intake gas pressure acquiring means for acquiring an intake gas pressure (Pim) in the intake manifold; an exhaust gas pressure acquiring means for acquiring an exhaust gas pressure (Pem) in the exhaust manifold; an intake gas flow acquiring means for acquiring an intake gas flow (Ga) which flows from the atmosphere; a fuel flow acquiring means for acquiring the fuel flow (Gf) supplied into the cylinders (2a-f); and an exhaust loss rate acquiring means for acquiring an exhaust loss rate, wherein the estimating means estimates the temperature increment (Tim'), which is caused by the adiabatic compression until a gas introduced in the intake manifold is exhausted to the exhaust manifold, by using the intake gas temperature (Tim) acquired by the intake gas temperature acquiring means, the intake gas pressure (Pim) acquired by the intake gas pressure acquiring means, and the exhaust gas pressure (Pem) acquired by the exhaust gas pressure acquiring means, and simultaneously estimates the temperature increment (ΔTf), which is caused by the amount of heat obtained by dividing the amount of heat (Qexf) lost in the exhaust gas among the amount of heat (Qf) introduced into the cylinders (2a-f) of the internal combustion engine by the diluted gas flow (Gexh) of the exhaust gas in the exhaust manifold by using the intake gas flow (Ga) acquired by the intake gas flow acquiring means, the fuel flow (Gf) acquired by the fuel flow acquiring means, and the exhaust loss rate acquired by the exhaust loss rate acquiring means, and wherein a map, which indicates the exhaust loss rate by using at least the number of revolutions of the engine and the fuel flow (Gf) as parameters, is prepared in advance, and the exhaust loss rate acquiring means acquires the exhaust loss rate by using the map. an estimating means for estimating the exhaust gas temperature (Tem), based on a temperature increment (Tim') caused by an adiabatic compression until a gas introduced into an intake manifold of the internal combustion engine is exhausted to the exhaust manifold, and a temperature increment (ΔTf) caused by an amount of heat obtained by dividing an amount of heat (Qexf) lost in an exhaust gas among an amount of heat (Qf) introduced into cylinders (2a-f) of the internal combustion engine by a diluted gas flow (Gexh) of the exhaust gas in the exhaust manifold; an intake gas temperature acquiring means for acquiring an intake gas temperature (Tim) in the intake manifold; an intake gas pressure acquiring means for acquiring an intake gas pressure (Pim) in the intake manifold; an exhaust gas pressure acquiring means for acquiring an exhaust gas pressure (Pem) in the exhaust manifold; an intake gas flow acquiring means for acquiring an intake gas flow (Ga) which flows from the atmosphere; a fuel flow acquiring means for acquiring the fuel flow (Gf) supplied into the cylinders (2a-f); and an exhaust loss rate acquiring means for acquiring an exhaust loss rate, wherein the estimating means estimates the temperature increment (Tim'), which is caused by the adiabatic compression until a gas introduced in the intake manifold is exhausted to the exhaust manifold, by using the intake gas temperature (Tim) acquired by the intake gas temperature acquiring means, the intake gas pressure (Pim) acquired by the intake gas pressure acquiring means, and the exhaust gas pressure (Pem) acquired by the exhaust gas pressure acquiring means, and simultaneously estimates the temperature increment (ΔTf), which is caused by the amount of heat obtained by dividing the amount of heat (Qexf) lost in the exhaust gas among the amount of heat (Qf) introduced into the cylinders (2a-f) of the internal combustion engine by the diluted gas flow (Gexh) of the exhaust gas in the exhaust manifold by using the intake gas flow (Ga) acquired by the intake gas flow acquiring means, the fuel flow (Gf) acquired by the fuel flow acquiring means, and the exhaust loss rate acquired by the exhaust loss rate acquiring means, and wherein a map, which indicates the exhaust loss rate by using at least the number of revolutions of the engine and the fuel flow (Gf) as parameters, is prepared in advance, and the exhaust loss rate acquiring means acquires the exhaust loss rate by using the map.