A mobile radiophone apparatus includes an A/D converter for outputting an input analogue voice signal as quantized data and a highly efficient voice encoder for extracting features of the quantized data output from the A/D converter and converting the data to codes of a smaller quantity by a fixed time unit. The number of quantization bits in the A/D converter is established to be larger than the number of quantization bits possible to be input to the highly efficient voice encoder. A bit-number converter is provided for converting the number of bits of the quantized data output at the A/D converter to the number of quantization bits possible to be input to the highly efficient voice encoder corresponding to a maximum amplitude value within the fixed time unit. the conversion parameter for the quantized data in the bit-number converter is transmitted in addition to the codes output at the highly efficient voice encoder.