PURPOSE: To prevent a malfunction due to dusts in air and to improve reliability by providing a dust collecting part which drops the dusts in air by bending the air stream on an air intake path.

CONSTITUTION: When an absorption fan 10 is driven, the air in a monitoring area 1 is introduced from a piping 20a to a dust collecting part 21. In the dust collecting part 21, the air stream is bent and the relatively heavy dust in the air is dropped and collected. The passed air is introduced into a dust collecting part 22 and passes through a filter 23 to collect the relatively small-sized dusts passing through the part 22 by the filter 23. That is, the dust collecting parts 21 and 22 are provided for removing the dusts in the air to be introduced into a fire detection part 9, and particulates of smoke whose particle sizes are smaller than the dust passes through the dust collecting parts 21 and 22 and introduced into a fire detection part 9. Thus, the part 9 can detect the accurate smoke density without being affected by the dusts in the monitoring area 1.