A gas mixture temperature estimation method for an internal combustion engine, for estimating a temperature (Tmix(k)) of a gas mixture forefront successively after injection of fuel, the gas mixture consisting of fuel injected into a combustion chamber of the internal combustion engine and dispersing progressively after the injection of fuel in a cylinder interior gas that is a gas having been taken into the combustion chamber, and a gas-mixture-forming cylinder interior gas being a portion of the cylinder interior gas and mixing with the injected fuel to produce the gas mixture, the amount (ma) of the gas-mixture-forming cylinder interior gas increasing progressively with progression of the dispersion of the injected fuel, the method comprising: estimating a traveling distance over which the gas mixture forefront travels from an injection opening successively after the injection of fuel; estimating the temperature (Tmix(k)) of the gas mixture forefront on the basis of the quantity of heat carried by the injected fuel and the quantity of heat carried by the gas-mixture-forming cylinder interior gas successively before the gas mixture forefront is determined to reach an inner wall surface of the combustion chamber after the injection of fuel based on the estimated traveling distance, under the assumption that heat transfer does not occur between the gas mixture and a peripheral cylinder interior gas that is a remaining portion of the cylinder interior gas other than the gas-mixture-forming cylinder interior gas and that exists around the gas mixture without mixing with the injected fuel; and estimating the quantity of heat transferred between the gas mixture and an object or substance existing around the gas mixture successively and estimating the temperature (Tmix(k)) of the gas mixture forefront on the basis of the quantity of heat carried by the injected fuel, the quantity of heat carried by the gas-mixture-forming cylinder interior gas, and the quantity of the heat transfer successively after the gas mixture forefront is determined to reach the inner wall surface of the combustion chamber based on the estimated traveling distance, under the assumption that the gas mixture stagnates in a generally annular configuration in the vicinity of a side wall of the combustion chamber and the heat transfer occurs between the gas mixture and the object or substance existing around the gas mixture. estimating a traveling distance over which the gas mixture forefront travels from an injection opening successively after the injection of fuel; estimating the temperature (Tmix(k)) of the gas mixture forefront on the basis of the quantity of heat carried by the injected fuel and the quantity of heat carried by the gas-mixture-forming cylinder interior gas successively before the gas mixture forefront is determined to reach an inner wall surface of the combustion chamber after the injection of fuel based on the estimated traveling distance, under the assumption that heat transfer does not occur between the gas mixture and a peripheral cylinder interior gas that is a remaining portion of the cylinder interior gas other than the gas-mixture-forming cylinder interior gas and that exists around the gas mixture without mixing with the injected fuel; and estimating the quantity of heat transferred between the gas mixture and an object or substance existing around the gas mixture successively and estimating the temperature (Tmix(k)) of the gas mixture forefront on the basis of the quantity of heat carried by the injected fuel, the quantity of heat carried by the gas-mixture-forming cylinder interior gas, and the quantity of the heat transfer successively after the gas mixture forefront is determined to reach the inner wall surface of the combustion chamber based on the estimated traveling distance, under the assumption that the gas mixture stagnates in a generally annular configuration in the vicinity of a side wall of the combustion chamber and the heat transfer occurs between the gas mixture and the object or substance existing around the gas mixture.