A misfire detecting apparatus is provided wherein a misfire is detected based on a rotational speed parameter indicative of a rotational speed of an internal combustion engine. An average change amount of the rotational speed parameter in a first predetermined period and an inertia speed changing component which is generated with rotation of the engine are calculated, and a first corrected rotational speed parameter is calculated by correcting the rotational speed parameter according to the average change amount and the inertia speed changing component. A first relative speed parameter is calculated according to a difference between a first reference value and the first corrected rotational speed parameter, the first reference value being the first corrected rotational speed parameter corresponding to the rotational speed parameter which is detected at a reference timing at which a piston of a cylinder, which is subjected to the misfire determination, is positioned in the vicinity of the compression top dead center. A first determination parameter is calculated by integrating the first relative speed parameter for an integration period corresponding to 720/N degrees of a crank angle (“N” is a number of cylinders of the engine), and a misfire determination is performed based on the first determination parameter. The reference timing and a start timing of the integration period are set according to the ignition timing of the engine.