To maintain a stable link, regardless of link rates to be used together in a PON (passive optical network) system, using a plurality of link rates together.

The PON system includes a station-side device 1 and a terminal-side device 2 for performing optical communication via single-core optical fibers 3, 5 and uses a plurality of link rates 1G, 10G, together in the optical communication between both devices 1 and 2. The PON system includes a rate selecting section 30 for selecting one from among the plurality of link rates; an RTT (round-trip time) correction section 32 for correcting an RTT between the station and the terminal, according to the selected link rate; and a link control section 31 for controlling a link using the corrected RTT at the link rate selected.