1. A method of generating LDPC (Low Density Parity Check) codes in a communication system using an error-correcting code, comprising the steps of: performing exponent conversion of a predetermined number of exponent matrixes stored in advance so as to generate new exponent matrixes based on said exponent conversion; and generating new LDPC codes using said new exponent matrixes, wherein said exponent conversion comprises: calculating the size of permutation matrixes constituting a given number of parity check matrixes; calculating exponent value of a parity check matrix for each of a given number of exponent matrixes; and performing exponent conversion of the size of said permutation matrixes and said exponent value.