A misfire detecting method in which misfire detection is carried out based on a rotating condition of an output shaft (7) of an internal combustion engine, which engine is controlled by a controller to which electric power is supplied from a power source for a time after an ignition key is turned off to permit the controller to conduct a control required at a time of stopping the engine operation. The method comprises the steps of: detecting (S4) when the ignition key is turned off; and prohibiting misfire detection until a predetermined detection-prohibiting time period has elapsed from the moment when it was detected that the ignition key had been turned off.