PURPOSE: To realize an ANC under the consideration of the detection error of a noise by a control sound without being affected by a transmitting characteristic between a noise detecting means and a control sound generating means.

CONSTITUTION: A duct 1 is constituted of first and second noise detecting means 3 and 4 which detect the noise of a noise source 2, control means which generates a control sound for canceling the noise based on the detection outputs of the first and second noise detecting means 3 and 4, control sound generating means 5 driven by the output of the control means, which generates the control sound, and error detecting means 6 which detects the noise canceled by the control sound. The control means is adjusted so that the detection output of the control sound by the first and second noise detecting means 3 and 4 can be substantially 0.