A misfire determination device for a multi-cylinder engine performing four strokes in one cycle, comprising - a rotor connected to a crankshaft of the engine and having a plurality of protrusions distanced from each other by a predetermined angle, - a crank position sensor which generates and outputs a pulse each time when each protrusion rotates by the predetermined angle, and - an engine microcomputer having - a timing means which times an elapsed time each time when the pulse is outputted from the crank position sensor into the engine microcomputer, wherein the elapsed time corresponds to a period of time until the pulses are counted, which are inputted into the engine microcomputer in one stroke, - a deviation calculating means which calculates a deviation between a first time, which represents the elapsed time when a current pulse is counted, and a second time, which represents the elapsed time when a pulse obtained prior to the current pulse is counted, characterized in that the engine microcomputer further having - an integrated value calculating means which calculates an integrated value each time when the pulse is outputted from the crank position sensor into the engine microcomputer, wherein the integrated value is obtained by integrating the deviation by the rotational angle corresponding to one stroke, - a determining means for determining whether a misfire occurs on the basis of a magnitude relationship between the integrated value and a determination value which serves as predetermined threshold value wherein the determination value is set on the basis of an optimal integrated value, - a counting means which increments a misfire determination number (Kn) by one when the determining means determine a misfire. - a rotor connected to a crankshaft of the engine and having a plurality of protrusions distanced from each other by a predetermined angle, connected to a crankshaft of the engine and having a plurality of protrusions distanced from each other by a predetermined angle, - a crank position sensor which generates and outputs a pulse each time when each protrusion rotates by the predetermined angle, and - an engine microcomputer having - a timing means which times an elapsed time each time when the pulse is outputted from the crank position sensor into the engine microcomputer, wherein the elapsed time corresponds to a period of time until the pulses are counted, which are inputted into the engine microcomputer in one stroke, - a deviation calculating means which calculates a deviation between a first time, which represents the elapsed time when a current pulse is counted, and a second time, which represents the elapsed time when a pulse obtained prior to the current pulse is counted, - a timing means which times an elapsed time each time when the pulse is outputted from the crank position sensor into the engine microcomputer, wherein the elapsed time corresponds to a period of time until the pulses are counted, which are inputted into the engine microcomputer in one stroke, - a deviation calculating means which calculates a deviation between a first time, which represents the elapsed time when a current pulse is counted, and a second time, which represents the elapsed time when a pulse obtained prior to the current pulse is counted, - an integrated value calculating means which calculates an integrated value each time when the pulse is outputted from the crank position sensor into the engine microcomputer, wherein the integrated value is obtained by integrating the deviation by the rotational angle corresponding to one stroke, - a determining means for determining whether a misfire occurs on the basis of a magnitude relationship between the integrated value and a determination value which serves as predetermined threshold value wherein the determination value is set on the basis of an optimal integrated value, - a counting means which increments a misfire determination number (Kn) by one when the determining means determine a misfire.