PURPOSE: To add a digital data transmission function to a voice encoded data block transmitter by adding an identification code ID to a digital data block and providing a means for surely detecting it.

CONSTITUTION: On a transmission side, when it is judged that the output of a voice encoding part 10 is silence in a silence judgement part 33, a transmission switching circuit 24 is switched to digital data transmission. An ID bit pattern for which a self correlation peak value is large is added to digital data read from a data butter 22 in an 10 addition part 32 and supplied to the switching circuit 24. In an ID discrimination part 36 on a reception side, the correlation function with the bit pattern at the leading parts of the respective data block is calculated and the ID of the data block is surely discriminated since a peak value is obtained where an input signal is the ID and the average value of mutual correlation becomes zero in the case of the leading part of voice encoded data. Thus, the digital data transmission function is easily added to the voice encoded data block transmitter.