An error correcting code decoding device includes a first decoding circuit, a word-length reduction circuit configured to reduce bit lengths of a first external values corresponding to a plurality of bits obtained after decoding process performed by the first decoding circuit a first predetermined number of times and to reduce bit lengths of words included in word string, and a second decoding circuit configured to decode the bit string by executing a decoding process a second predetermined number of times for calculating second external values and posterior values of the bits included in the bit string in accordance with the word string including the words having the reduced bit lengths using the first external values having the reduced bit lengths as second prior probabilities that corresponding bits among the plurality of bits are the predetermined value.