PURPOSE: To reduce the packet storage capacity by allowing a packet storage section to write information to an optional address from which recorded information is read at the very moment when the information is read therefrom and using the addresses circulatingly to avoid information in excess of bit number of a packet from being inputted.

CONSTITUTION: Information is written to an optional address from which recorded information is read in a RAM 30 of a packet storage section 25 at the very moment when the information is read therefrom, storage areas of the RAM 30 are used circulatingly so that the information in excess of M-bits is not inputted to the RAM 30. Thus, the capacity of the RAM 30 is enough to be M bits at minimum. Thus, the capacity of the packet storage section 25 is made small and the component cost is reduced.