An error correcting decoding apparatus of an extended RS code capable of solving a problem of a conventional method in that Euclidean algorithm or Berlekamp-Massey algorithms must be performed twice in the worst case because of complicated algorithm, and this results in a delay of decoding. The present apparatus generates a syndrome from a received word, estimates the number of errors having occurred in the received word, computes error-locator polynomials and error-value polynomials while changing the initial values and ending condition of the Euclidean algorithm computation in accordance with the number of errors estimated, computes error locations and error values by performing Chien's search on these polynomials, and carries out the error correction on the basis of the error locations and error values. This makes it possible to achieve decoding by performing the Euclidean algorithm computation only once.