A dehumidification air conditioner reduces carbon dioxide levels in a low humidity workroom. Cooling dehumidification is performed on outdoor air in a pre-air-cooler to produce pre-cooled air, which is branched such that a first part passes through a processing zone of an adsorption rotor which can remove carbon dioxide and humidity simultaneously, and a second part passes through a purge zone of the adsorption rotor. Air which passed through the processing zone is supplied to a low humidity workroom. Air which passed though the purge zone of the adsorption rotor is mixed with outdoor air and then heated with a reproduction heater to produce heated air. The heated air is sent to a reproduction zone of the adsorption rotor, to simultaneously remove carbon dioxide and humidity from the adsorption rotor and produce an exhaust stream which exhausted out of the device.