A misfire detecting apparatus for an internal combustion engine, having rotational speed parameter detecting means for detecting a rotational speed parameter indicative of a rotational speed of said engine, and detecting a misfire of said engine based on the detected rotational speed parameter, the misfire detecting apparatus comprising: first correcting means for calculating a first average change amount (DELTAOMGTDCA/4π) of the rotational speed parameter in a first predetermined period, calculating an inertia speed change component (OMGI) which is generated with rotation of said engine, and calculating a first corrected rotational speed parameter (OMGMA) by correcting the rotational speed parameter according to the average change amount (DELTAOMGTDCA/4π) and the inertia speed change component (OMGI); first relative speed parameter calculating means for calculating a first relative speed parameter (OMGREFA) according to a difference between a first reference value (OMGMATDC) and the first corrected rotational speed parameter (OMGMA), the first reference value (OMGMATDC) being the first corrected rotational speed parameter (OMGMA) corresponding to the rotational speed parameter which is detected at a reference timing (IGOFST) 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; first determination parameter calculating means for calculating a first determination parameter (MFPARAMA) by integrating the first relative speed parameter (OMGREFMA) for an integration period (TINTG) corresponding to 720/N degrees of a crank angle, where "N" is a number of cylinders of said engine; and determining means for performing a misfire determination based on the first determination parameter (MFPARAMA), characterized in that the reference timing (IGOFST) and a start timing (CAIS) of the integration period (TINTG) are set according to the ignition timing (IGLOG) of said engine, so that a retard amount of the reference timing (IGOFST) is increased as a retard amount of the ignition timing (IGLOG) increases. first correcting means for calculating a first average change amount (DELTAOMGTDCA/4π) of the rotational speed parameter in a first predetermined period, calculating an inertia speed change component (OMGI) which is generated with rotation of said engine, and calculating a first corrected rotational speed parameter (OMGMA) by correcting the rotational speed parameter according to the average change amount (DELTAOMGTDCA/4π) and the inertia speed change component (OMGI); first relative speed parameter calculating means for calculating a first relative speed parameter (OMGREFA) according to a difference between a first reference value (OMGMATDC) and the first corrected rotational speed parameter (OMGMA), the first reference value (OMGMATDC) being the first corrected rotational speed parameter (OMGMA) corresponding to the rotational speed parameter which is detected at a reference timing (IGOFST) 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; first determination parameter calculating means for calculating a first determination parameter (MFPARAMA) by integrating the first relative speed parameter (OMGREFMA) for an integration period (TINTG) corresponding to 720/N degrees of a crank angle, where "N" is a number of cylinders of said engine; and determining means for performing a misfire determination based on the first determination parameter (MFPARAMA), characterized in that the reference timing (IGOFST) and a start timing (CAIS) of the integration period (TINTG) are set according to the ignition timing (IGLOG) of said engine, so that a retard amount of the reference timing (IGOFST) is increased as a retard amount of the ignition timing (IGLOG) increases.