An apparatus for defecting occurrence of misfiring in an internal combustion engine with improved reliability. A change in an angular velocity of a crankshaft of the engine and a change in the output of an air/fuel ratio sensor are monitored. Only when both changes indicate occurrence of misfiring independently from each other, it is decided that misfiring in the cylinder has taken place. Reliability in misfiring detection is improved by using the results obtained through at least two mutually different types of misfiring detections.