A communication system including a second communication device which sends data to a first communication device, and also including a third communication device which sends data to said second communication device, the communication system comprising: a buffer unit which is provided in said second communication device and temporarily stores data sent from said third communication device for the purpose of sending the data to said first communication device; a data measurement unit which is provided in said second communication device and measures the volume of data stored in said buffer unit; a sending data storage unit which is provided in said third communication device and stores data to be sent to said second communication device; a transmission unit which is provided in said third communication device and sends data stored in said sending data storage unit to said second communication device; and a congestion state determination unit which is provided in said second or third communication devices (200, 300) and, on the basis of a measurement value of said data volume measured by said data measurement unit, determines whether or not said buffer unit is in a congestion state; wherein, on the basis of a result of the determination by said congestion state determination unit, said transmission unit sends data stored in said sending data storage unit to said second communication device; wherein said second communication device is provided with a congestion state notification unit which, when said congestion state determination unit has determined that said buffer unit is in a congestion state, notifies said third communication device of said buffer unit being in the congestion state, by including congestion state information in an S1 application protocol, S1-AP, or a GPRS tunneling protocol for user plane, GTP-u, message or parameter; and said transmission unit sends data stored in said sending data storage unit to said second communication device, on the basis of said congestion state information sent by said congestion state notification unit. a buffer unit which is provided in said second communication device and temporarily stores data sent from said third communication device for the purpose of sending the data to said first communication device; a data measurement unit which is provided in said second communication device and measures the volume of data stored in said buffer unit; a sending data storage unit which is provided in said third communication device and stores data to be sent to said second communication device; a transmission unit which is provided in said third communication device and sends data stored in said sending data storage unit to said second communication device; and a congestion state determination unit which is provided in said second or third communication devices (200, 300) and, on the basis of a measurement value of said data volume measured by said data measurement unit, determines whether or not said buffer unit is in a congestion state; wherein, on the basis of a result of the determination by said congestion state determination unit, said transmission unit sends data stored in said sending data storage unit to said second communication device; wherein said second communication device is provided with a congestion state notification unit which, when said congestion state determination unit has determined that said buffer unit is in a congestion state, notifies said third communication device of said buffer unit being in the congestion state, by including congestion state information in an S1 application protocol, S1-AP, or a GPRS tunneling protocol for user plane, GTP-u, message or parameter; and said transmission unit sends data stored in said sending data storage unit to said second communication device, on the basis of said congestion state information sent by said congestion state notification unit.