A radio base station (eNB) is configured to start the transmitting-side timer and a Polling timer which is set to expire earlier than the transmitting-side, upon transmission of a last piece of downlink data accumulated in a transmission buffer to the mobile station. When the Polling timer expires, the radio base station (eNB) notifies the mobile station (UE) of the expiration, and restarts the transmitting-side timer, and the mobile station (UE) restarts a receiving-side timer in response to the notification.