A transmission controller monitors a sound pressure determination signal and a distance determination signal. The transmission controller controls a transmission voice processor to start an operation of generating a transmission voice signal, when the distance determination signal indicates that a distance is equal to or less than a first distance. The transmission controller controls to start an operation of determining a sound pressure of a voice signal, when the distance determination signal indicates that the distance is equal to or less than a second distance shorter than the first distance. The transmission controller supplies a transmission control signal to a transmission circuit so that the transmission circuit transmits the transmission voice signal as a radio wave, when the sound pressure determination signal indicates that the sound pressure is equal to or greater than a predetermined threshold value.