PURPOSE: To improve the noise performance and the sound quality and to prevent howling, by using a notch filter attenuating the sensitivity of the resonance frequency of the frequency characterstics and frequencies in the vicinity thereof.

CONSTITUTION: An audio signal of the user outputted from an ear microphone (EM)1 is transmitted to other radio device via a radio device 3, outputted as a voice from a speaker 4 or inputted to a voice input device 5. A filter 2 comprises a high pass filter 2a forming a frequency characteristic A, a low pass filter 2b forming a frequency characteristic B and the notch filter 2c forming the notch characteristic of the resonance frequency of about 4kHz of the frequency characteristic of the EM1 and frequencies in its vicinity. The audio signal extracted from the EM1 has a frequency characteristic C through the filter 2, and the frequency characteristic where the uniform sensitivity is given to the resonance frequency of about 4kHz and the frequency in its vicinity, is attained.