judgment of vibration i e combustion noise of the internal combustion engine 100 based on the output signals in the present embodiment an output signal of the crank angle sensor 10 is a rectangular pulse of voltage a pulse rise with the voltage rising is defined as a rise and a pulse fall with the voltage falling is defined as a fall a given number z of pulses for one cycle will be output repeatedly within the ecu 1 a reference point k a is determined at the rise timing of each pulse suffix a denotes a reference point k number e g reference point k a 1 denotes a reference point determined at the rise of a first pulse after a predetermined period elapsed after the output of the z th pulse of the given number z of pulses the last one of the given number z of pulses is denoted as a z a rotation angle of the crankshaft sensed for a period elapsed between reference points k 1 and k 2 is denoted as rotation angle ca 2 of the crankshaft a rotation angle of the crankshaft sensed for a period until each next reference point k a determined other than the above period is denoted as rotation angle ca 1.