An air composition adjustment device for adjusting composition of air in a target space , comprising: a humidifier for humidifying a target space ; and a composition adjuster that includes a separating unit that separates, from air-to-be-treated, supply air having a composition different from a composition of the air-to-be-treated, wherein the humidifier comprises: an air path through which water-extraction air flows; a cooling unit that cools the water-extraction air in the air path ; a water extracting unit that extracts water that is generated when the water-extraction air is cooled by the cooling unit in the air path ; and a humidifying unit that humidifies the target space by using the water extracted by the water extracting unit , wherein the air path includes a main path that communicates with the target space and a discharge path configured to discharge a part or all of the water-extraction air to the outside of the target space , wherein the main path of the air path includes a path through which the air-to-be-treated flows to the separating unit and a path through which the supply air generated in the separating unit flows to the target space , wherein the cooling unit is configured to cool and dehumidify the air-to-be-treated or the supply air in the air path , wherein the water extracting unit is configured to extract water generated by cooling the air-to-be-treated or the supply air that flows through the air path , and wherein the humidifying unit is configured to humidify the target space by using the water extracted by the water extracting unit . a humidifier for humidifying a target space ; and a composition adjuster that includes a separating unit that separates, from air-to-be-treated, supply air having a composition different from a composition of the air-to-be-treated, wherein the humidifier comprises: an air path through which water-extraction air flows; a cooling unit that cools the water-extraction air in the air path ; a water extracting unit that extracts water that is generated when the water-extraction air is cooled by the cooling unit in the air path ; and a humidifying unit that humidifies the target space by using the water extracted by the water extracting unit , an air path through which water-extraction air flows; a cooling unit that cools the water-extraction air in the air path ; a water extracting unit that extracts water that is generated when the water-extraction air is cooled by the cooling unit in the air path ; and a humidifying unit that humidifies the target space by using the water extracted by the water extracting unit , wherein the air path includes a main path that communicates with the target space and a discharge path configured to discharge a part or all of the water-extraction air to the outside of the target space , wherein the main path of the air path includes a path through which the air-to-be-treated flows to the separating unit and a path through which the supply air generated in the separating unit flows to the target space , wherein the cooling unit is configured to cool and dehumidify the air-to-be-treated or the supply air in the air path , wherein the water extracting unit is configured to extract water generated by cooling the air-to-be-treated or the supply air that flows through the air path , and wherein the humidifying unit is configured to humidify the target space by using the water extracted by the water extracting unit . <EOS>