This study describes the early and reliable fire detection in air-conditioned rooms. It is important to detect fires in their earlier stages as possible, but young fi rest weak signals reaching fire detectors are generally indistinguishable from those by non-fire sources. Moreover, the flows due to air-conditioners equipped in recent buildings highly affect the fire gases in rooms, which increases the difficulty in early and reliable fire detection. The objective of this study is to investigate the interactions between fire gases and air-conditioning flow in rooms in relation to the effectiveness of multi-sensor fire detectors including the carbon monoxide sensor. First described are the experimental results of fire .gas flow in rooms with and without air-conditioner. And next made are numerical studies to visually examine the flow patterns of fire gases in air-conditioned rooms. It has been found that air-conditioners equipped in rooms greatly affect the flow patterns in rooms and cause the dilution of not only the smoke and fire gases but also the cigarette smoke and bath steam which often cause false alarms. Therefore the recognition of fires or non-fires in the air-conditioned rooms must be made in lower sensor signal levels than the traditional ones, so multi-sensor fire detectors including the carbon monoxide sensor are highly effective to detect fires earlier and reliably in air-conditioned rooms.