removing an echo from an input signal input from a microphone of a terminal including a speaker and the microphone;

converting the echo-removed signal into a function of a frequency domain;

estimating a noise signal included in an echo-removed signal as a signal generated by removing the echo from the input signal to generate an estimated noise signal;

suppressing the noise signal included in the echo-removed signal based on the estimated noise signal to generate a suppressed signal;

generating a comfort noise based on the estimated noise signal;

superimposing the comfort noise on the suppressed signal,

converting a signal generated by a superimposition unit into a function of a time domain,

generating the estimated noise signal for each frequency band based on the echo-removed signal converted into the function of the frequency domain by the converter,

generating the comfort noise for each frequency band based on the estimated noise signal for each frequency band,

wherein the superimposition unit uses the comfort noise as a threshold value and superimposes the comfort noise on the suppressed signal only in a frequency band in which the magnitude of the suppressed signal falls below the comfort noise.