PURPOSE: To make detection high speed and high sensitivity by obtaining power of signals before removing echo by an echo canceler and power of signals after removing echo by integration and detecting double talk from the power ratio.

CONSTITUTION: Sensitivity of detection of double talk is heightened by operating equations (5), (6), (7) using (ej) of large variation corresponding to change of n(t). Here, K is maximum value of power ratio of y(t) and n(t) when detection of double talk is necessary, and (q) is integer coefficient used in equations (6), (7) and both of them can be set to proper values. Power calculation of equations (6), (7) can be executed by two times of multiplication, subtraction, shifting and addition using necessary memories for two data. Thus, as power of (ej) and n(t) becomes nearly equal when an echo canceler is converged sufficiently, when the power of n(t) increases, (ej) also increases, and double talk can be detected quickly from the inequality (5).