a signal amplifier (11) for amplifying signals for discriminating the broadcasting state; a peak value detecting section (12) for detecting the peak values of the output signals of the amplifier (11); a biasing section (16) for supplying biasing voltages to both the amplifier (11) and to the peak voltage detecting section (12); a logic-combining circuit section (13) for logic-combining the positive and negative output signals of the peak voltage detecting section (12); an automatic level control section (14) for maintaining the peak value of the output level of the amplifier (11) at a constant value; and a flip-flop (15) for being set by the negative signals of the peak value detecting section (12), and for being reset by the positive signals thereof.
