The present invention relates to a demodulation method for enhancing decoding performance of information bits in a constant-amplitude multi-code biorthogonal modulation communication system, which performs encoding to cause the level of a transmission symbol to be constant. In the demodulation method of the present invention, a demodulator demodulates the received signal, the demodulator having four blocks. C (c≦2k, k is the number of information bits) bit streams with higher correlation values in descending order are selected and combined among demodulated bit streams to establish c4 bit stream candidates. A parity check is performed with respect to the c4 bit stream candidates, a candidate group having a maximal sum of correlation values is selected among candidate groups in which any error is not generated at the parity check. A bit stream corresponding to the selected candidate group is determined to be an output of a receiver. If a hypothesis in which any error is not generated does not exist, a bit stream determined using a maximum correlation value with respect to each of the blocks is output. According to the present invention, there is an advantage in that an error in reception bits can be detected or corrected, thus improving reception performance.