The internal combustion engine system of the invention calculates a time constant T of a high pass filter for eliminating a resonance component of a damper as a distortional element from a rotation speed Ne of an engine, and sets the high pass filter with computation of a transfer function from the time constant T (steps S110 and S120). The set high pass filter is applied to a 30-degree rotation time T30 representing a rotational variation of the engine to obtain a filtered 30-degree rotation time F30 with elimination of the resonance component of the damper 28 (step S130). The occurrence of an engine misfire is detected, based on a 30-degree rotation time difference D30 and a misfire detection base difference J30 computed from the filtered 30-degree rotation times F30 (step S140). 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 Ne of the engine.