There is provided a relay server 20 relaying communications between terminals using an application. Each terminal 10 includes a voice call means 11 which performs a voice call process, and a voice call information transmission means 12 which, when the voice call process is performed by the voice call means 11, transmits to the relay server 20 a voice call status change notice inclusive of information for identifying the terminal 10 and indicative of voice call status having been changed, the transmission being performed by communication using the application. Upon receipt of communication data through communication using the application from a communication source terminal 10, the relay server 20 identifies a communication destination terminal based on the voice call status change notice and transmits the communication data to the identified communication destination terminal.