An example operating method of an error correction code (ECC) circuit includes receiving a codeword from a memory device, calculating a syndrome vector based on the codeword and a parity-check matrix indicating whether messages are exchanged between check nodes and variable nodes, performing, when the syndrome vector is not a zero vector, sequential decoding on a plurality of columns of the parity-check matrix by decoding a first column in a first operation mode, the first column having a first variable node degree, decoding a second column in a second operation mode, the second column having a second variable node degree, and decoding a third column in a third operation mode, the third column having a third variable node degree, and calculating the syndrome vector whenever the sequential decoding of the plurality of columns is completed and iteratively performing the sequential decoding until the syndrome vector is the zero vector.