PURPOSE: To abort a packet of non-small missing class with priority over a packet of small missing class without use of a special FIF0 memory by reading and outputting a packet from a 2nd FIFO memory only when a stored packet number in a 1st FIFO memory exceeds a prescribed number.

CONSTITUTION: When a stored packet number in a 1st FIFO memory 13 in which a packet is written independently of priority of abort is less than a preset threshold value, the 1st FIFO memory 13 is selected and its packet is outputted to an output terminal 12. When the stored packet number exceeds the preset threshold value, a 2nd FIFO memory 14 in which only packets of small missing class having a tight requirement to abort are written is selected and its packet is outputted to the output terminal 12. Thus, a packet of non-small missing class is aborted with priority over a packet of small missing class without use of a special FIFO memory.