An optical line terminal (OLT) that configures a passive optical network (PON) system in cooperation with an optical network unit (ONU). The OLT has a controller to control a link state with the ONU. The controller generates, in one case, a control signal specifying a first low power consumption mode in which the ONU operates in the low power consumption mode in an upstream direction, and generates, in another case, the control signal specifying a second low power consumption mode in which the ONU operates in the low power consumption mode in both upstream and downstream directions. Further, the generated control signal is transmitted to the ONU.