An air conditioning system comprising: an air supply fan configured to supply outside air into a room; an air supply pipe serving as a path for conveying outside air to the room; a heater configured to heat outside air before being introduced into the air supply pipe or outside air in the air supply pipe on an upstream side of the air supply pipe; a humidity sensor configured to measure a humidity of outside air before being introduced into the air supply pipe, a humidity of outside air in the air supply pipe, or a humidity of outside air led out of the air supply pipe; and a control unit configured to control the air supply fan and the heater, wherein the control unit is configured to change alternatively from a first air-supplying operation of heating outside air with the heater and supplying the heated outside air into the room, in accordance with a concentration of a predetermined gas in the room; to a second air-supplying operation of supplying outside air into the room without heating the outside air, in accordance with the concentration of the predetermined gas in the room; and wherein the control unit is configured to select the first air-supplying operation on condition that the humidity measured by the humidity sensor has a value equal to or more than a predetermined humidity value, and to select the second air-supplying operation on condition that the humidity measured by the humidity sensor has a value less than the predetermined humidity value. an air supply fan configured to supply outside air into a room; an air supply pipe serving as a path for conveying outside air to the room; a heater configured to heat outside air before being introduced into the air supply pipe or outside air in the air supply pipe on an upstream side of the air supply pipe; a humidity sensor configured to measure a humidity of outside air before being introduced into the air supply pipe, a humidity of outside air in the air supply pipe, or a humidity of outside air led out of the air supply pipe; and a control unit configured to control the air supply fan and the heater, wherein the control unit is configured to change alternatively from a first air-supplying operation of heating outside air with the heater and supplying the heated outside air into the room, in accordance with a concentration of a predetermined gas in the room; to a second air-supplying operation of supplying outside air into the room without heating the outside air, in accordance with the concentration of the predetermined gas in the room; and a first air-supplying operation of heating outside air with the heater and supplying the heated outside air into the room, in accordance with a concentration of a predetermined gas in the room; to a second air-supplying operation of supplying outside air into the room without heating the outside air, in accordance with the concentration of the predetermined gas in the room; and wherein the control unit is configured to select the first air-supplying operation on condition that the humidity measured by the humidity sensor has a value equal to or more than a predetermined humidity value, and to select the second air-supplying operation on condition that the humidity measured by the humidity sensor has a value less than the predetermined humidity value.