Abstract: An apparatus for controlling output shaft torque of an engine in an engine tester by correction inertia of the dynamometer, comprising an operation pattern generator, a dynamometer processing unit, a dynamometer controller, and a dynamometer. The torque signal fed back to the dynamometer controller is not from the shaft torque meter but is of the dynamometer torque, and is not affected by the torsional vibration, and is controlled with good stability. Further, no time lag caused by the inertia of the dynaomometer develops in controlling the torque of the output shaft. Therefore, the torque of the output shaft can be controlled with high response and accuracy.