A dynamic bandwidth allocation apparatus 12 of the present invention is mounted on, for example, an optical line terminal 1 in a PON system that relays upstream frames F1 and F2 received from optical network units 2, to upper networks 6. The dynamic bandwidth allocation apparatus 12 calculates, based on the reception rates of the upstream frames F1 and F2 from the optical network units 2, the sending rates of relay destinations of the upstream frames F1 and F2, and changes in the amounts of data occurring upon relaying the upstream frames F1 and F2, allocated bandwidth upper limits (max_bw) at which even when the amounts of data increase, the upstream frames F1 and F2 can be relayed; and dynamically allocates amounts of upstream transmission for the respective optical network units 2 in the range of the calculated allocated bandwidth upper limits (max_bw).