PURPOSE: To simplify a computing method by a method wherein a change amount of a cylinder internal pressure at a given crank angle is determined, and the change amount is corrected by means of a correction factor responding to a change in a stroke volume during an explosion stroke to compute an indicated average effective pressure.

CONSTITUTION: By means of signals from a cylinder internal pressure sensor 2 and a crank angle sensor 3, a cylinder internal pressure detecting circuit 4 detects a cylinder internal pressure at each given crank angle, for example, 30°CA. A factor computing circuit 5 computes a correction factor by means of which a cylinder internal pressure at a crank angle symmetrical to a compression top dead center TDC is corrected to output it to a deviation computing circuit 6. The deviation computing circuit 6 computes a deviation between cylinder internal pressures at respective given crank angles to output a computing result to a total sum circuit 7. The total sum circuit 7 computes a total sum at a given crank angle section of computing results, inputted from the deviation computing circuit 6, as a current indicated average effective pressure (torque). As a result, an indicated average effective pressure in a conventional computer for controlling an engine can be computed, and a cost can be reduced.