1. An error correcting method comprising: a syndrome generating step for generating four syndromes S0, S1, S2 and S3 from a received word; an equation generating step for generating three equations A, B and C based on the syndromes, EQU A=S0S2+S1.sup.2 EQU B=S1S2+S0S3 EQU C=S1S3+S2.sup.2 ; an estimating step for estimating a number of errors in the received word, based on the syndromes and the equations such that it is estimated that no error has occurred in the received word when all of the syndromes are zero, that a single error has occurred in the received word when all of the equations are zero, and that a double error has occurred in the received word when B is not zero; a calculating step for calculating an error position and an error value when the single error or the double error has occurred; and a correcting step for correcting the error based on the position and value calculated in said calculating step.