PURPOSE: To inform a user of deterioration of the communication state while the communication is carried out between a radio base station and a digital radio telephone system carried by the user.

CONSTITUTION: A digital radio telephone system carried by a user includes an error rate calculating part 11 which calculates the error rate of the digital signal demodulated at a receiving part and a warning tone replacing part 12 which replaces the digital signal of a fixed section including a part demodulated by mistake with another digital signal produced by coding previously the warning tone signal. When the part 11 detects a fact that the error rate of the digital signal exceeds a fixed level, the part 12 replaces the digital signal of a fixed section including a part demodulated by mistake with another digital signal obtained by coding previously the warning tone signal and makes the user hear the voices mixed with a warning tone.