A method for detecting a misfire by fluctuation in crankshaft rotation in which the occurrence of a misfire can be detected exactly by eliminating the effect of detection error in detecting the fluctuation in crankshaft rotation, caused by the error in the construction of a crank angle sensor. By succeedingly receiving pulses from the crank angle sensor, a processor detects a time interval taken from the entering into a crankshaft rotation angle region, relating to a cylinder corresponding to both pulses, to the leaving therefrom (S2), calculates the crankshaft angular acceleration D.omega..sub.n by using the correction factor calculated on the basis of the above detected time interval, and the time interval determined by dividing the time taken for one rotation of the crankshaft by the number of cylinders (S3, S4), the latter interval being free from sensor error, and detects the occurrence of a misfire when the calculated value D.omega..sub.n is less than the decision value (S5 through S7).