an optical line terminal (OLT);

a plurality of optical network units (ONU) connected to the OLT through a passive optical network (PON) respectively; and

a gateway (GW) connected to the OLT through a communication line;

wherein the OLT includes:

a congestion control table that records a relationship between an output port number of the GW and an identifier (ID) of an ONU that uses the GW output line identified with the output port number;

an OLT control part that records a transmission queue length notified in an upstream PON frame from each of the plurality of ONUs beforehand and allocates a bandwidth distributed to each of the plurality of ONUs dynamically as the next upstream frame transmission bandwidth in a PON section according to its transmission queue length; and

a downstream frame generation part that generates a downstream frame including bandwidth control information indicating an upstream frame transmission bandwidth for each ONU according to a frame transmission bandwidth allocated by the OLT control part,

wherein the OLT control part, when receiving a congestion occurrence notice indicating a number of the output port in which congestion occurred (congestion occurred output port number) from the GW, identifies the ONU identifier corresponding to the congestion output port number in the bandwidth control table and shifts the current PON section bandwidth control in a normal mode for allocating a bandwidth to that in a bandwidth suppression mode for allocating a congestion time allowable bandwidth that is less than the current bandwidth to an ONU having the identified ONU ID and allocating a bandwidth to each of other ONUs according to its transmission queue length.