Work done by an engine can be accurately calculated regardless of the part in an observation section where the cylinder internal pressure signal is detected. The apparatus for calculating the work done by an engine establishes in advance correlation of phase between the cylinder internal pressure of the engine and a reference signal composed of a predetermined frequency component as a reference phase relation. A means for detecting the cylinder internal pressure of the engine for a predetermined observation section is provided. A reference signal corresponding to the detected cylinder internal pressure of the engine is calculated so that the reference phase relation is satisfied. A correlation coefficient of the detected cylinder internal pressure of the engine and the calculated reference signal is calculated for the observation section and the work done by the engine is calculated in accordance with the correlation coefficient.