An in-vehicle terminal and a mobile terminal receive a request and a first identifier for identifying the request and store the first identifier in a buffer, the mobile terminal sends the request to a server, which stores the first identifier in a buffer when the request includes the first identifier and generates a second identifier when the request does not include the first identifier. When an event occurs, the mobile terminal generates and stores a message and a message code related to the second identifier together with the second identifier, transfers a response including the first identifier, the message and the message code to the mobile terminal when the first identifier exists in the buffer, and transfers a response including the message and the message code to the mobile terminal when the first identifier does not exist in the buffer. The mobile terminal displays the message and the message code.