Air-conditioning system and communication method

A communication method executed by an air conditioning system including first air conditioning devices belonging to a first refrigerant group and second air conditioning devices belonging to a second refrigerant group, the first air conditioning devices and the second air conditioning devices being connected so as to be capable of communication with each other, the communication method comprising:

• executing encrypted communication between the first air conditioning devices, the encrypted communication involving encryption of communication data with a common key;
• executing the encrypted communication between the second air conditioning devices;
• and
• executing the encrypted communication between a first relay device among the first air conditioning devices and a second relay device among the second air conditioning devices, when a first communication device among the first air conditioning devices communicates with a second communication device among the second air conditioning devices.