Abstract: An anti-howl back device which includes various devices and a CPU that controls these devices, their decisions, and their processings in order to output the signals which are fed into a selector circuit upon the removal of acoustic feedback components in a manner that makes a decision on presence of a synchronous signal and an address signal in combination in the output from the selector circuit. Upon the affirmative result of such a decision, an A-D converter carries out an A-D conversion of the sample obtained by a sampling performed in synchronism with a synchronous signal and then subtracts the data which has been fetched from an address in a memory system corresponding to the address signal in combination with the synchronous signal from the data derived as a result of the A-D conversion. The new data thus obtained is then stored in the memory system while being concurrently delivered to a D-A converter.