Provided is an air conditioning system that maintains a room pressure in a clean room at high accuracy while having a simple configuration. An air conditioning system includes: a fan filter unit including a supply air fan to supply air to a preparation room from a chamber including space above a ceiling of the preparation room; a fan filter unit including a supply air fan to supply the air to the preparation room from the chamber; and a fan filter unit including a return air fan to return the air from the preparation room through the chamber and to exhaust the air from the preparation room, and further includes: a pressure sensor provided in the preparation room; and a control unit to control a return air fan based on a value detected by the pressure sensor.