First, “Sync+Wakeup ID” is transmitted form a first vehicle (from a first time to a second time). While a first portable device paired with the first vehicle does not respond to this, when another second portable device has an inverted code coinciding by chance, the second portable device transmits a “Welcome” response (from the second time to a fifth time). During this response, a “Who” command including proper “Sync+Wakeup ID” and “command+additional data” is transmitted, thereby allowing the first portable device to normally respond to “Who” (a sixth time).