A communication system according to the present invention includes an application server (10) and a relay apparatus (20) that relays a communication between the application server (10) and a communication network (40). The application server (10) comprises a communication unit (11) that transmits to the relay apparatus (20) a first message in which a first identifier has been set as a destination, the first identifier identifying a communication terminal (30) that performs communication via the communication network (40). The relay apparatus (20) includes a converter (21) that converts the first identifier into a second identifier that uniquely identifies the communication terminal (30) in the communication network (40), and a communication unit (22) that transmits a second message in which the second identifier has been set as a destination to the communication terminal (30) via the communication network (40).