The internal combustion engine system of the invention calculates a time constant of a high pass filter for eliminating a resonance component of a damper as a distortional element from a rotation speed of an engine, and sets the high pass filter with computation of a transfer function from the time constant. The set high pass filter is applied to a 30-degree rotation time representing a rotational variation of the engine to obtain a filtered 30-degree rotation time with elimination of the resonance component of the damper. The occurrence of an engine misfire is detected, based on a 30-degree rotation time difference and a misfire detection base difference computed from the filtered 30-degree rotation times. This arrangement ensures highly-accurate detection of the occurrence of a misfire in the engine constructed to output power via the damper as the distortional element, irrespective of the rotation speed of the engine.