Flexible printed wiring substrate

A method of mounting a flexible substrate having a branch structure onto a terrace-like substrate jutting from one end of an optical transmission module, wherein

• the flexible substrate includes:
• a connector;
• a first body containing first electric wiring connected to the connector, and including a plurality of solderable first electrodes;
• and a second body containing second electric wiring connected to the connector, and including at least one side portion which extends from the connector in a first direction being a longitudinal direction of the flexible substrate, and a transverse side portion which extends from the at least one side portion in a second direction being approximately perpendicular to the first direction, and which includes a plurality of solderable second electrodes,
• the second body is formed in a band-like form to surround the first body,
• the first body and the second body are joined together at least in or near a region to which the connector is connected, and
• a formation region of the plurality of first electrodes and a formation region of the plurality of second electrodes are located at approximately the same positions in the second direction, and
• the method comprises the steps of:
• soldering the plurality of first electrodes of the first body to electrodes provided on one surface of the terrace-like substrate;
• folding back the second body at an intermediate part of the at least one side portion toward the connector, and shaping the second body such that the second electrodes and electrodes provided on an opposite surface of the terrace-like substrate are located at the same position;
• and
• soldering the plurality of second electrodes of the second body to the electrodes on the opposite surface of the terrace-like substrate.