Method of operating a drainless air conditioner that performs an air conditioning process on mixed air, which is a mixture of some indoor air and outdoor air, the air conditioner comprising: a main body casing; an outdoor air supplying unit, which takes the outdoor air into the main body casing and deliver such to the indoor space; an indoor air supplying unit, which delivers to the indoor space, as return air, some of the indoor air taken into the main body casing; an exhausting unit, which delivers to the outdoor space, as exhaust air, the indoor air other than the return air that was taken into the main body casing; a first adsorptive heat exchanger, which has an adsorbing material configured to adsorb and desorb moisture in air, that functions as an adsorber that adsorbs the moisture or as a regenerator that restores the adsorbed moisture to the air and that is configured to perform a humidity conditioning process on the air; a second adsorptive heat exchanger, which has an adsorbing material configured to adsorb and desorb moisture in the air, that functions as an adsorber that adsorbs the moisture or as a regenerator that restores the adsorbed moisture to the air and that is configured to perform a humidity conditioning process on the air; and an air heat exchanger, configured to exchange heat between the air and a fluid after it has undergone a heat treatment by a heat source; wherein, the main body casing houses the first adsorptive heat exchanger, the second adsorptive heat exchanger, the air heat exchanger, the outdoor air supplying unit, the indoor air supplying unit, and the exhausting unit; wherein the method comprises the steps of: subjecting the outdoor air to a humidity conditioning process by the first adsorptive heat exchanger or the second adsorptive heat exchanger and mixing the outdoor air with some of the indoor air to become the mixed air; exchanging heat between the mixed air and the fluid by the air heat exchanger; subjecting the exhaust air to the humidity conditioning process by the first adsorptive heat exchanger or the second adsorptive heat exchanger and subsequently delivering it to the outdoor space; wherein when the first adsorptive heat exchanger functions as an adsorber, the second adsorptive heat exchanger functions as a regenerator; and when the first adsorptive heat exchanger functions as a regenerator, the second adsorptive heat exchanger functions as an adsorber. a main body casing; an outdoor air supplying unit, which takes the outdoor air into the main body casing and deliver such to the indoor space; an indoor air supplying unit, which delivers to the indoor space, as return air, some of the indoor air taken into the main body casing; an exhausting unit, which delivers to the outdoor space, as exhaust air, the indoor air other than the return air that was taken into the main body casing; a first adsorptive heat exchanger, which has an adsorbing material configured to adsorb and desorb moisture in air, that functions as an adsorber that adsorbs the moisture or as a regenerator that restores the adsorbed moisture to the air and that is configured to perform a humidity conditioning process on the air; a second adsorptive heat exchanger, which has an adsorbing material configured to adsorb and desorb moisture in the air, that functions as an adsorber that adsorbs the moisture or as a regenerator that restores the adsorbed moisture to the air and that is configured to perform a humidity conditioning process on the air; and an air heat exchanger, configured to exchange heat between the air and a fluid after it has undergone a heat treatment by a heat source; subjecting the outdoor air to a humidity conditioning process by the first adsorptive heat exchanger or the second adsorptive heat exchanger and mixing the outdoor air with some of the indoor air to become the mixed air; exchanging heat between the mixed air and the fluid by the air heat exchanger; subjecting the exhaust air to the humidity conditioning process by the first adsorptive heat exchanger or the second adsorptive heat exchanger and subsequently delivering it to the outdoor space; wherein when the first adsorptive heat exchanger functions as an adsorber, the second adsorptive heat exchanger functions as a regenerator; and when the first adsorptive heat exchanger functions as a regenerator, the second adsorptive heat exchanger functions as an adsorber.