To maintain a satisfactory sound quality by analyzing the frequency characteristics of a sound in real time, automatically selecting an optimum sound band on the basis of the analyzed result and transmitting the audio signal of the selected band after converting it to a frequency band to be transmitted by communication equipment.

A sound analytic part 2 analyzes the frequency characteristics of an input audio signal 1. According to the analyzed result of the sound analytic part 2, a sound band selecting part 3 automatically selects the optimum sound band. A frequency converting part 4 converts the selected sound band to the band transmissible for radio equipment and inputs it to the radio equipment. Thus, although actually transmitted audio information is changed corresponding to an individual or clause and the sound quality on the side of reception is changed corresponding to the individual or sound contents since audio frequency characteristics are fluctuated based on individual pronunciation and clause, even when the audio frequency band is remarkably fluctuated by the individual difference of pronunciation or clause, the optimum sound band can be selected at all the time.