A misfiring detector for detecting a misfire occurring in an internal combustion engine which comprises a gas current pulse generator which has electrodes forming an ion-gap for generating gas current pulse signal when a high temperature burned gas is emitted through the ion-gap from the engine, an engine speed sensor for generating an engine speed pulse signal with a repetition rate proportional to the speed of the engine, and an electronic computing means responsive to the gas current pulse signal and the engine speed pulse signal to produce an electric output signal representing the occurrence of the misfire when the ratio of the number of the gas current pulse signal to that of the engine speed pulse signal is less than a predetermined value. The gas current pulse generator is disposed in an exhaust manifold in the vicinity of an exhaust port as adjacently as possible. The misfiring detector further comprises an indicator for indicating the occurrence of the misfire and a storing means for storing the misfiring condition. Furthermore, disclosed herein is a protection system for preventing an exhaust gas purifying means from being destroyed by unburned gas caused by the misfire through utilization of the above misfiring detector. The protection is accomplished by shutting off the unburned gas and/or a secondary air drawn into the exhaust gas purifying means in the event that the misfire occurs in the engine, or by controlling an amount of the secondary air.