PURPOSE: To incarnate comfortable sound environment, in which a window, etc., are opened and fresh air flows in, but external noises do not intrude, in living space.

CONSTITUTION: An opening-closing sensor 19 generates an open-state detecting signal KD and transmits it over a control circuit for a signal processing section 24 when a vestibule door is opened. Consequently, noise suppressing operation is started. A sensor microphone 7 detects external noises reaching a vestibule opening section, and converts them into a noise signal EN. An error microphone 10 detects the level of residual noises, which are denied by additional sounds emitted from noise suppressing speakers 5a, 5b and damp and remain, and converts it into a residual noise signal SN. The signal processing section 24 generates an additional sound signal for emitting additional sounds opposite in phase to external noises at the same sound pressure as external noises on the basis of the noise signal EN and the residual noise signal SN, and transmits it over the noise suppressing speakers 5a, 5b.