The present invention provides a coupling mechanism between the optical subassembly and the circuit board using the FPC board. The mechanism according to the present invention may release the mechanical stress induced in the root portion of the lead pin of the optical subassembly and shows a superior productivity. The optical assembly provides a plurality of lead pins arranged in at least two rows. The FPC board bent in the U-shape is put between rows and soldered to the lead pins in the pads thereof. One piece of the FPC board is extended to the circuit board and soldered thereto.