PURPOSE: To allow high-compression coding by subjecting the data of every period of the parts having the periodicity of voice signals to ADPCM coding.

CONSTITUTION: The contents of the mode data for the purpose of coding are set at 0 indicating a soundless coding mode when the execution of a coding program is executed. Whether the contents of the mode are the coding mode (PT) based on the Fourier transform of a pitch period or not is decided. A 'not' is decided as the mode is 0. The decision of the ADPCM coding mode is likewise the 'not'. The level Lin of the input voice signal S is compared with a reference level signal L1. The soundless state is decided when the level is smaller than L1. The S is coded by the soundless coding mode and is stored in a RAM 8. The content of the mode is then set in an AD and when all of three sets of the data in the signal S are above the reference level L2, the ADPCM coding is executed and the coding data is stored into the RAM 8. The voice signal waveform is then subjected to the Fourier transform. The coding by the PT is executed when the mode is decided to be the PT.