An air-conditioning system includes: a first duct through which air discharged from an electronic instrument passes; a water vapor retention chamber; an evaporator configured to be heated by the air passing through the first duct and thereby to generate water vapor inside the water vapor retention chamber; a desiccant rotor including an adsorbent to adsorb moisture and configured to be driven and rotated by a driving unit; a humidification chamber; a second duct configured to feed outdoor air introduced from an outdoor space into the humidification chamber; and a third duct configured to feed the air passing through the humidification chamber into a room where the electronic instrument is installed, wherein a portion of the desiccant rotor is located inside the water vapor retention chamber, and another portion of the desiccant rotor is located inside the humidification chamber.