In the code word mapping operation of a radio communication system, mapping patterns are provided for different S/N ratios, the code word bits produced from a coder are not equally assigned to multi-level modulation bits, but weighted according to the resistance of multi-level modulation bits to error before being assigned, and the mapping patterns are switched in accordance with S/N. Since the code word mapping method is updated so that the error rate can be always minimized according to the situations of a propagation path and S/N ratio, communication can be made with high communication quality.