Abstract: A communication adapter connected to an indoor unit included in an air conditioner includes a communication unit, a determination unit, and a communication control unit. The communication unit communicates data to an air conditioning control apparatus that performs centralized control of a plurality of air conditioners. The determination unit determines whether or not transmission target data is communicated by using encryption communication. The communication control unit controls, when the transmission target data is communicated by using the encryption communication, the communication unit in order to communicate the transmission target data by using the encryption communication. Consequently, it is possible to decrease a laying cost of a dedicated communication cable and reduce a communication load needed for data communication related to the air conditioners in an existing communication network at the time of central control performed on the air conditioners.