An internal combustion engine system having a multi-cylinder internal combustion engine constructed to output power to a driveshaft via a distortional element being a damper, the internal combustion engine system comprising: a rotational position detector configured to detect a rotational position of an output shaft of the internal combustion engine; a rotation speed detector configured to detect a rotation speed of the internal combustion engine; a rotational variation processor configured to compute a rotational variation of the internal combustion engine from the detected rotational position; an influencing component eliminator configured to set a time constant of a high pass filter used in an influencing component elimination process, which applies the high pass filter to the rotational variation to cut off a resonance frequency domain of the distortional element to eliminate an influencing component on the rotational variation of the internal combustion engine by resonance of the distortional element, based on the detected rotation speed of the internal combustion engine, where the high pass filter having a cutoff frequency set to be not lower than a specific frequency at a rate of one to two rotations of the output shaft of the internal combustion engine, and to apply the influencing element elimination process using the high pass filter with the set time constant to the computed rotational variation, so as to obtain an influencing component-free rotational variation with elimination of the influencing component on the rotational variation of the internal combustion engine by the resonance of the distortional element; and an engine misfire detector configured to detect occurrence of a misfire in the internal combustion engine, based on the influencing component-free rotational variation, wherein the influencing component eliminator sets a transfer function of the high pass filter at a sampling cycle, which is set equal to a rotational variation of the internal combustion engine at intervals of a preset rotational angle, based on the detected rotation speed. a rotational position detector configured to detect a rotational position of an output shaft of the internal combustion engine; a rotation speed detector configured to detect a rotation speed of the internal combustion engine; a rotational variation processor configured to compute a rotational variation of the internal combustion engine from the detected rotational position; an influencing component eliminator configured to set a time constant of a high pass filter used in an influencing component elimination process, which applies the high pass filter to the rotational variation to cut off a resonance frequency domain of the distortional element to eliminate an influencing component on the rotational variation of the internal combustion engine by resonance of the distortional element, based on the detected rotation speed of the internal combustion engine, where the high pass filter having a cutoff frequency set to be not lower than a specific frequency at a rate of one to two rotations of the output shaft of the internal combustion engine, and to apply the influencing element elimination process using the high pass filter with the set time constant to the computed rotational variation, so as to obtain an influencing component-free rotational variation with elimination of the influencing component on the rotational variation of the internal combustion engine by the resonance of the distortional element; and an engine misfire detector configured to detect occurrence of a misfire in the internal combustion engine, based on the influencing component-free rotational variation, wherein the influencing component eliminator sets a transfer function of the high pass filter at a sampling cycle, which is set equal to a rotational variation of the internal combustion engine at intervals of a preset rotational angle, based on the detected rotation speed.