1. A misfire detecting apparatus for an internal combustion engine, comprising: a) reference angular position setting means (2) mounted on a crank shaft (14) of said internal combustion engine and including a plurality of segments (12,13) individually defining angular intervals corresponding to reference positions for a plurality of cylinders of said engine, respectively; b) a crank angle sensor (3) disposed in opposition to said reference angular position setting means for generating a plurality of pulse signals as an angular position signal (SGT) for every predetermined crank angle per one rotation of said crank shaft; c) period measuring means (6) for measuring periods (T) of said plurality of pulse signals, respectively; d) correcting coefficient arithmetic means (7) for arithmetically determining a plurality of correcting coefficients (KL1,KL2) individually corresponding to the plurality of segments on the basis of said plurality of periods in a steady operation state of said internal combustion engine; e) period variation index arithmetic means (8) for arithmetically determining an index (.alpha.o) which quantifies variations of said periods on the basis of said correcting coefficients and said periods; and f) misfire decision means (9) for making a decision as to an occurrence of misfire in said internal combustion engine on the basis of said period variation index; g) wherein said correcting coefficient arithmetic means includes updating arithmetic means for updating said plurality of correcting coefficients to respective latest values by learning; and h) wherein said updating arithmetic means updates said plurality of correcting coefficients such that a number of times for updating said correcting coefficients is substantially the same for each one of said plurality of correcting coefficients, respectively.