A ventilation device for an air conditioning system, wherein the air conditioning system also includes, in addition to the ventilation device, air conditioners providing air conditioning to a room, the ventilation device comprising: a casing that includes an air-supply passage and an exhaust passage; an air-supply blower that is provided in the air-supply passage, and that is configured to blow outdoor air into the air-supply passage to form a supplied-air flow in the room; an exhaust blower that is provided in the exhaust passage, and that is configured to blow indoor air into the exhaust passage to form an exhaust-air flow to be discharged outside the room; a total heat exchanger that is located between the air-supply passage and the exhaust passage, and accommodated in the casing, and that is configured to perform total heat exchange between the supplied-air flow and the exhaust-air flow; an outside-air temperature sensor for measuring a temperature of the outdoor air; an outside-air humidity sensor for measuring a humidity of the outdoor air; an indoor humidity sensor for measuring a humidity of the indoor air; a temperature regulating coil that is capable of changing a heating capacity at multiple stages, and that heats the supplied-air flow after having undergone total heat exchange by the total heat exchanger; a humidifying element that is configured to humidify a supplied-air flow heated by the temperature regulating coil; a target indoor-humidity storage unit that is configured to store therein a target indoor humidity that is a target value of a humidity of the indoor air; and a control unit; wherein the control unit is configured to determine a heating capacity of the temperature regulating coil, such that a humidity of the supplied-air flow becomes the target indoor humidity, based on measurement values of the outside-air temperature sensor and the outside-air humidity sensor when an actual measurement value of a humidity of the indoor air measured by the indoor humidity sensor is equal to or higher than the target indoor humidity the ventilation device being characterized in that: the control unit is configured to change the target indoor humidity based on number of the air conditioners in the air conditioning system, which are performing a cooling operation, or the control unit is configured to change the target indoor humidity based on a refrigerant evaporating temperature in the air conditioners. a casing that includes an air-supply passage and an exhaust passage; an air-supply blower that is provided in the air-supply passage, and that is configured to blow outdoor air into the air-supply passage to form a supplied-air flow in the room; an exhaust blower that is provided in the exhaust passage, and that is configured to blow indoor air into the exhaust passage to form an exhaust-air flow to be discharged outside the room; a total heat exchanger that is located between the air-supply passage and the exhaust passage, and accommodated in the casing, and that is configured to perform total heat exchange between the supplied-air flow and the exhaust-air flow; an outside-air temperature sensor for measuring a temperature of the outdoor air; an outside-air humidity sensor for measuring a humidity of the outdoor air; an indoor humidity sensor for measuring a humidity of the indoor air; a temperature regulating coil that is capable of changing a heating capacity at multiple stages, and that heats the supplied-air flow after having undergone total heat exchange by the total heat exchanger; a humidifying element that is configured to humidify a supplied-air flow heated by the temperature regulating coil; a target indoor-humidity storage unit that is configured to store therein a target indoor humidity that is a target value of a humidity of the indoor air; and a control unit; wherein the control unit is configured to determine a heating capacity of the temperature regulating coil, such that a humidity of the supplied-air flow becomes the target indoor humidity, based on measurement values of the outside-air temperature sensor and the outside-air humidity sensor when an actual measurement value of a humidity of the indoor air measured by the indoor humidity sensor is equal to or higher than the target indoor humidity the ventilation device being characterized in that: the control unit is configured to change the target indoor humidity based on number of the air conditioners in the air conditioning system, which are performing a cooling operation, or the control unit is configured to change the target indoor humidity based on a refrigerant evaporating temperature in the air conditioners. the control unit is configured to change the target indoor humidity based on number of the air conditioners in the air conditioning system, which are performing a cooling operation, or the control unit is configured to change the target indoor humidity based on a refrigerant evaporating temperature in the air conditioners.