A combustion state diagnostic apparatus configured to diagnose a combustion state of fuel in a compression self-igniting internal combustion engine that is configured to carry out combustion by self-ignition of the fuel injected from a fuel injection valve into a cylinder, the combustion state diagnostic apparatus of the internal combustion engine comprising: wherein it is configured that a reaction velocity, a reaction amount, and a reaction period of a respective plurality of reactions of the fuel injected into the cylinder are calculated in accordance with an in-cylinder environment, and an ideal heat generation rate waveform model, or an ideal heat generation rate waveform produced by performing filter processing for the ideal heat generation rate waveform model, with regard to each reaction, is respectively obtained, and the ideal heat generation rate waveform model or the ideal heat generation rate waveform with regard to each reaction is respectively compared with an actual heat generation rate waveform of a case where the fuel is actually reacted in the cylinder, and when there exists a reaction in which deviation of the actual heat generation rate waveform with respect to the ideal heat generation rate waveform model or the ideal heat generation rate waveform is equal to or higher than a predetermined amount, it is diagnosed that abnormality occurs in the reaction; wherein the respective plurality of reactions of the fuel are a gasification reaction, a low-temperature oxidation reaction, a thermal decomposition reaction, and a high-temperature oxidation reaction, and in each of the respective reactions, it is configured to compare the ideal heat generation rate waveform model or the ideal heat generation rate waveform with the actual heat generation rate waveform and diagnose the abnormality of the reaction; wherein a reaction start temperature is set as a reaction starting point of each of the respective reactions, and a time point when an in-cylinder temperature reaches the reaction start temperature is set as a start time of the reaction; and wherein the ideal heat generation rate waveform model is produced by being approximated as a triangle in which the start time of each reaction is provided as a reference point, and a gradient of an oblique side represents the reaction velocity, and an area represents the reaction amount, and a length of a base represents the reaction period. wherein it is configured that a reaction velocity, a reaction amount, and a reaction period of a respective plurality of reactions of the fuel injected into the cylinder are calculated in accordance with an in-cylinder environment, and an ideal heat generation rate waveform model, or an ideal heat generation rate waveform produced by performing filter processing for the ideal heat generation rate waveform model, with regard to each reaction, is respectively obtained, and the ideal heat generation rate waveform model or the ideal heat generation rate waveform with regard to each reaction is respectively compared with an actual heat generation rate waveform of a case where the fuel is actually reacted in the cylinder, and when there exists a reaction in which deviation of the actual heat generation rate waveform with respect to the ideal heat generation rate waveform model or the ideal heat generation rate waveform is equal to or higher than a predetermined amount, it is diagnosed that abnormality occurs in the reaction; wherein the respective plurality of reactions of the fuel are a gasification reaction, a low-temperature oxidation reaction, a thermal decomposition reaction, and a high-temperature oxidation reaction, and in each of the respective reactions, it is configured to compare the ideal heat generation rate waveform model or the ideal heat generation rate waveform with the actual heat generation rate waveform and diagnose the abnormality of the reaction; wherein a reaction start temperature is set as a reaction starting point of each of the respective reactions, and a time point when an in-cylinder temperature reaches the reaction start temperature is set as a start time of the reaction; and wherein the ideal heat generation rate waveform model is produced by being approximated as a triangle in which the start time of each reaction is provided as a reference point, and a gradient of an oblique side represents the reaction velocity, and an area represents the reaction amount, and a length of a base represents the reaction period.