In a method for measuring variations in the output of an internal combustion engine, at least one of signals respectively representing an engine speed, torque, and internal cylinder pressure of the internal combustion engine during the firing strokes of the engine is obtained. A periodical and pulsating change in the obtained signal is detected. A differential value at a portion of the change of the signal having a maximum gradient is then detected. Another method provides measuring variations in the output of the internal combustion engine by calculating the difference between the maximum and minimum values of the change of the obtained signal. Still another method provides measuring variations in the output of the internal combustion engine by correcting the obtained value in accordance with the engine speed. Still another method provides measuring variations in the output of the internal combustion engine by calculating the difference between consecutive changes.