A signal transmission method in a radio multiplex transmission system that includes serial-to-parallel converting of serial data to be transmitted into N parallel data series and independently performing an error-correcting encoding process on the parallel signals of the N data series. The method further includes parallel-to-serial converting of the parallel signals encoded with error-correcting codes, performing an interleaving process on the parallel-to-serial converted signals and serial-to-parallel converting the interleaved signals into L parallel data series and transmitting each of the L data series. The transmitted signals are then received and separated into M data series and are parallel-to-serial converted and a deinterleaving process is performed. The method also includes serial-to-parallel converting the deinterleaved signals into N data series, independently performing an error-correcting decoding process on the parallel signals of the N data series serial-to-parallel converted, and parallel-to-serial converting the signals in which the error-correcting codes are decoded.