Each of the plurality of queues stores packet data of a received packet. The read concession assignor assigns one of the plurality of queues with a read concession for a predefined time period. The overdraft storage stores an overdraft amount in connection with each of the plurality of queues. The read adequacy determiner determines, in accordance with an overdraft amount stored in connection with one queue out of the plurality of queues, whether to read packet data from the one queue. The overdraft updater updates at least one of a first overdraft amount stored in connection with a first queue and a second overdraft amount stored in connection with a second queue different from the first queue upon reading packet data from the first queue during a time period while the second queue is assigned with the read concession.