An operation method of a flash memory system includes a hard decision decoding on a codeword and a soft decision decoding on an error message block. The hard decision decoding on a codeword and the codeword comprises message blocks encoded with row constituent codes and column constituent codes according to a block-wise concatenated BCH (BC-BCH) method. When the hard decision decoding fails, the error message block to which the hard decision decoding fails among a plurality of the message blocks is identified. Soft decision information corresponding to the row constituent codes and the column constituent codes of the error message block is generated and the soft decision decoding on the error message block based on the soft decision information is performed.