Provided is a method of forming a parity-check matrix for a parallel concatenated LDPC code, wherein the parallel concatenated LDPC code is composed of a first LDPC code, a second LDPC code, and an interleaver connecting therebetween. The method includes: the steps of (a) finding a degree distribution of a first LDPC code and a degree distribution of a second LDPC; and (b) forming the parity-check matrices of the first and second LDPC codes satisfying the degree distributions, wherein in the (a) step, the degree distributions of the first and second LDPC codes are found using performance measurement by a density evolution method, and wherein in the density evolution method, the probability density of a message forwarded from a variable node of the first LDPC code to a check node reflects a probability density of extrinsic information outputted from the second LDPC code, and the probability density of a message forwarded from a variable node of the second LDPC code to a check node reflects a probability density of extrinsic information outputted from the first LDPC code.