To surely suppress howling in a loudspeaking system in a simple configuration.

A delay unit 2 and an adaptive filter 3 delay a microphone input signal y(n) to generate a sine component b(n)=sin(ω0*t+θ'+θ") delayed from the howling component sin(ω0*t+θ) by 2nπ and a subtractor 4 is used to subtract the component bn from the microphone input signal y(n) to suppress the howling.