The present invention relates to a pressure detecting apparatus which detects the peak of combustion pressure in a cylinder of an internal combustion engine and the timing of that peak. The apparatus according to the present invention can be constructed in two alternative ways. That is, in one way, once stored data of combustion pressure in the cylinder is read out in reverse order, thus the difference between the read-out data and the data of combustion pressure in the cylinder is obtained, and the peak of combustion pressure in the cylinder and the timing of the peak are detected in accordance with the difference. In another way, the inflection points of the differentiated result of the data of combustion pressure in the cylinder are detected, and the peak of combustion pressure and the timing of the peak are detected in accordance with the inflection points. Consequently, the peak of combustion pressure in the cylinder and the timing of the peak can be detected easily even in the case where the internal combustion engine is idling and the peak of combustion pressure in the cylinder has comb to be less than the maximum pressure value of the motoring, for example. As a result, it is possible to accurately detect the combustion condition in the cylinder of the internal combustion engine.