[Problem] To make a data storage capacity variable and to increase or reduce the storage area in a device mounted in a communication device, without stopping the communication device. [Solution] A dynamic variable capacity memory device 40 is mounted in a plurality of communication devices 20a-20d that communicate with each other via a bidirectional path, and is configured from an electronic circuit that is a gate array, which has a variable circuit configuration is capable of storing data. The dynamic variable capacity memory device 40 comprises a storage unit in which there is used a storage circuit (FPGA) based on an electronic circuit that is a gate array capable of storing data. The dynamic variable capacity memory device 40 is also configured to comprise a control unit that: establishes an allotment that corresponds to a data capacity transmittable through a path between the communication devices 20a-20d; and performs a capacity variable control for forming, with a storage circuit, a storage area of storage capacity for storing data of the corresponding path in the storage unit, in accordance with the allotment.