To early detect that a filter is closed by clogging, by installing a detecting part which detects foreign matter, and a differential pressure detecting part which detects the difference between the atmospheric pressure in a detection space to a filter and the atmospheric pressure in the outside which is obtained via a penetrating hole.

The air in a room is introduced into a detection space 123 from a filter 121. Whether carbon monoxide exists in the air in the detection space 123 is monitored by a CO sensor 122. When carbon monoxide is detected, a current is outputted to a circuit board 14, and a buzzer 17 is operated. The clogging of the filter 121 is detected on the basis of atmospheric pressure difference between the detection space 123 and the inside of the room. When the clogging exists, atmospheric pressure difference is generated between the inside of the room and the detection space 123. By the difference, a diaphragm in a differential pressure detection part 13 is differentially operated, so that difference is generated in a capacitance. Hence a differential pressure sensor 13 detects the clogging of the filter 121, drives the buzzer 17, and gives the alarm.