A signal processing method for a digital signal comprising the steps of: establishing a Yule-Walker equation having the following form by using a matrix that includes, as components, the elements of a Galois field GF(2.sup.m), and a vector that includes, as components, the elements of the Galois field GF(2.sup.m).times..times..LAMBDA..LAMBDA..times. ##EQU00001## employing Jacobi's formula to obtain the solution of the above equation by the calculation of determination of symmetric matrices; determining the number of errors to be the maximum matrix size that corresponds to the obtained solution that is not zero; and determining whether the number of errors equals the maximum number of correctable errors.