A rectangular array of 6.times.150 bits is divided into 50 rectangular arrays of 8.times.3 bits with 2 check bits added, resulting in 50 codes C.sub.2. This encoding is executed 50 times to form a rectangular array of 8.times.150 bits and each of its rows is encoded into a code C.sub.1 of 166 bits with 16 check bits added. This encoding is executed eight times to form a codeword of a generalized product code, which includes 8.times.166 bits and each row following 12 synchronizing bits. Errors are detected in the codes C.sub.1 decoded from the codeword to produce erasure information which is utilized to correct the code C.sub.2. Then, the original array is reproduced.