A device for estimating pressure and temperature of gas in a gas passage provided in an internal combustion engine, comprising: first estimation means for estimating a time-derivative dM/dt of a mass of gas in the gas passage in accordance with an equation of a relation obtained through application of a mass conservation law to the gas in the gas passage, the equation being dM / dt = mt − mc where mt represents the mass flow rate of gas flowing into the gas passage; mc represents the mass flow rate of gas flowing out of the gas passage; M represents the mass of gas in the gas passage; and t represent time; second estimation means for estimating a time-derivative dTm/dt of a temperature of the gas in the gas passage in accordance with an equation of a relation obtained through application of an energy conservation law to the gas in the gas passage, the equation being dTm / dt = 1 / M ⋅ Cv ⋅ mt ⋅ Cp ⋅ Ta − mc ⋅ Cp ⋅ Tm − dM / dt ⋅ Cv ⋅ Tm where mt represents the mass flow rate of gas flowing into the gas passage; mc represents the mass flow rate of gas flowing out of the gas passage; M represents the mass of gas in the gas passage; Ta represents the temperature of the gas flowing into the gas passage; Tm represents the temperature of the gas in the gas passage; Cv represents the specific heat at constant volume of the gas in the gas passage; Cp represents the specific heat at constant pressure of the gas in the gas passage; and t represents time; and third estimation means for obtaining the mass of gas through iterative integration of the estimated time-derivative of the mass of the gas with respect to time, for obtaining the temperature of the gas through iterative integration of the estimated time-derivative of the temperature of the gas with respect to time, and for estimating the pressure of the gas in the gas passage using the obtained mass of the gas and the obtained temperature of the gas, and in accordance with a gas state equation which is applied to the gas in the gas passage and which includes a term regarding the effective volume of the gas passage. first estimation means for estimating a time-derivative dM/dt of a mass of gas in the gas passage in accordance with an equation of a relation obtained through application of a mass conservation law to the gas in the gas passage, the equation being dM / dt = mt − mc where mt represents the mass flow rate of gas flowing into the gas passage; mc represents the mass flow rate of gas flowing out of the gas passage; M represents the mass of gas in the gas passage; and t represent time; second estimation means for estimating a time-derivative dTm/dt of a temperature of the gas in the gas passage in accordance with an equation of a relation obtained through application of an energy conservation law to the gas in the gas passage, the equation being dTm / dt = 1 / M ⋅ Cv ⋅ mt ⋅ Cp ⋅ Ta − mc ⋅ Cp ⋅ Tm − dM / dt ⋅ Cv ⋅ Tm where mt represents the mass flow rate of gas flowing into the gas passage; mc represents the mass flow rate of gas flowing out of the gas passage; M represents the mass of gas in the gas passage; Ta represents the temperature of the gas flowing into the gas passage; Tm represents the temperature of the gas in the gas passage; Cv represents the specific heat at constant volume of the gas in the gas passage; Cp represents the specific heat at constant pressure of the gas in the gas passage; and t represents time; and third estimation means for obtaining the mass of gas through iterative integration of the estimated time-derivative of the mass of the gas with respect to time, for obtaining the temperature of the gas through iterative integration of the estimated time-derivative of the temperature of the gas with respect to time, and for estimating the pressure of the gas in the gas passage using the obtained mass of the gas and the obtained temperature of the gas, and in accordance with a gas state equation which is applied to the gas in the gas passage and which includes a term regarding the effective volume of the gas passage.