A method of calculating the amount of work done by an internal combustion engine, which is capable of calculating a work amount parameter indicative of the amount of work done with accuracy while compensating for a phase delay in in-cylinder pressure data caused e.g. by a filtering process performed thereon. The method calculates the work amount parameter Pmi indicative of the amount of work done by the engine 3 including a combustion chamber 3b, as follows: Pressure in the combustion chamber 3b is detected to obtain in-cylinder pressure data P indicative of the detected pressure. Volume data V indicative of a state of change in volume of the combustion chamber 3b is obtained. The obtained volume data V is subjected to a first predetermined filtering process. The work amount parameter Pmi is calculated based on the detected in-cylinder pressure data P, and the volume data filtered value VF obtained by subjecting the volume data V to the first filtering process.