In a wireless base station device, a PDCP buffer unit temporarily stores data sent from a core network device for the purpose of sending the data to a mobile station device. A data measurement unit measures the volume of data stored in a PDCP buffer unit. On the basis of a measurement value of the data volume measured by the data measurement unit, the congestion state determination unit determines whether or not the PDCP buffer unit is in a congestion state. In the core network device, a sending data storage unit stores data to be sent to the wireless base station device. On the basis of a result of the determination by the congestion state determination unit, a transmission/reception unit sends the data stored in the sending data storage unit to the wireless base station device. In this way, loss of data communicated between the communication devices is reduced.