A station-side apparatus in an optical transmission system, for transferring frames between a host apparatus and a plurality of subscriber-side apparatuses connected via first to Nth (N ≥ 2) optical splitters (2-0 to 2-N -1), comprising: first to Nth optical transceivers (11-0 to 11-N- 1) that are connected to the first to Nth optical splitters (2-0 to 2-N -1) in one-to-one correspondence, and each are adapted to perform electro-optical conversion of a downstream electrical signal into an optical signal to be output to the subscriber-side apparatuses and photoelectric conversion of an optical signal from one of the subscriber-side apparatuses into an upstream electrical signal; the station-side apparatus being characterized in a selection and distribution circuit that is adapted to select one of the first to Nth optical transceivers (11-0 to 11-N- 1) and to output an upstream electrical signal input from the selected one of the optical transceivers while distributing and outputting the downstream electrical signal to the first to Nth optical transceivers (11-0 to 11-N -1); and a control circuit that is adapted to control the plurality of subscriber-side apparatuses not to transmit upstream frames at the same time while controlling the selection and distribution circuit, the selection and distribution circuit including a selection control unit that is adapted to select one of the first to Nth optical transceivers (11-0 to 11-N -1) based on a timing of a discovery window for waiting for reception of a registration request frame from an unregistered subscriber-side apparatus, a timing of a grant as a reception period of an upstream frame from a registered subscriber-side apparatus, and a logical link identification number for a logical link with the registered subscriber-side apparatus assigned to the grant, all of which are transmitted from the control circuit. first to Nth optical transceivers (11-0 to 11-N- 1) that are connected to the first to Nth optical splitters (2-0 to 2-N -1) in one-to-one correspondence, and each are adapted to perform electro-optical conversion of a downstream electrical signal into an optical signal to be output to the subscriber-side apparatuses and photoelectric conversion of an optical signal from one of the subscriber-side apparatuses into an upstream electrical signal; the station-side apparatus being characterized in a selection and distribution circuit that is adapted to select one of the first to Nth optical transceivers (11-0 to 11-N- 1) and to output an upstream electrical signal input from the selected one of the optical transceivers while distributing and outputting the downstream electrical signal to the first to Nth optical transceivers (11-0 to 11-N -1); and a control circuit that is adapted to control the plurality of subscriber-side apparatuses not to transmit upstream frames at the same time while controlling the selection and distribution circuit, the selection and distribution circuit including a selection control unit that is adapted to select one of the first to Nth optical transceivers (11-0 to 11-N -1) based on a timing of a discovery window for waiting for reception of a registration request frame from an unregistered subscriber-side apparatus, a timing of a grant as a reception period of an upstream frame from a registered subscriber-side apparatus, and a logical link identification number for a logical link with the registered subscriber-side apparatus assigned to the grant, all of which are transmitted from the control circuit.