A demodulation apparatus and a communication system using the same are disclosed. The apparatus comprises code generation means for generating a reference spread code corresponding to a first signal modulated by a spread code, and a surface acoustic wave element for receiving the first signal and a second signal output from the code generation means, and outputting a demodulated information signal. The surface acoustic wave element comprises a piezoelectric substrate, a first excitation electrode, formed on the piezoelectric substrate, for generating a first surface acoustic wave which propagates in a predetermined direction in accordance with the first signal, a second excitation electrode, formed on the piezoelectric substrate, for generating a second surface acoustic wave which propagates in a direction opposite to the predetermined direction in accordance with the second signal, and an acousto-electric converter, formed between the first and second excitation electrodes on the piezoelectric substrate, for outputting a convolution signal of the first and second signals on the basis of the first and second surface acoustic waves, wherein the acousto-electric converter selectively converts a surface acoustic wave having a wave number twice a wave number of the first and second surface acoustic waves into an electrical signal.