A write preference determination unit (30A) compares a reception rate of packets received from the lines of a first network (NW1) with a reception rate threshold for write preference determination, and in a case where the reception rate exceeds the reception rate threshold, determines that preference of a write operation is necessary. A write preference control unit (30B) increases, out of a total access bandwidth of a packet buffer (BUF), a write bandwidth for a packet write operation to the packet buffer (BUF) as compared to a read bandwidth for a packet read operation from the packet buffer (BUF) in a case where the write preference determination unit (30A) determines that the preference is necessary, thereby preferentially executing the packet write operation to the packet buffer. This suppresses occurrence of linked discard of reception packets caused by a shortage of the write bandwidth.