Customer premises optical network unit and optical transmission system

An optical network unit according to the present invention is provided as comprising a configuration that component units built therein are grouped for at least two sheets of substrate modules and arranged thereat. There are provided individual embodiments: (a) arranging an L2 layer and a part of the component unit of an L1 layer at a first substrate module, meanwhile, arranging the left part of the component unit of the L1 layer at a second substrate module; (b) arranging the component units of the L1 layer and of the L2 layer at the first substrate module and the second substrate module individually by grouping therefor; and (c) arranging the component units of the L2 layer and of the L1 layer at the first substrate module and the second substrate module respectively.