PURPOSE: To eliminate a need for the retrieval of an idle buffer missing queue at the time of connecting a buffer to the queue, which lacks an idle buffer, and to eliminate the preparation of a command form at the time of completing reception preparation and informing the transmission side of it.

CONSTITUTION: When data are received in the buffer by a communicating means 1 and there is no idle buffer in a queue even at the time of further newly connecting any idle buffer to the queue when there is no idle buffer, the number of the queue lacking the idle buffer is stored in a storage means as above- mentioned 2. When the idle buffer is newly prepared, a connecting means 3 reads the number of the idle buffer lacking queue registered on this storage means 2 and connects the idle buffer to the reception queue.