The present invention relates to an optical transmission module comprising two substrates, each of which has an electronic part mounted on the substrate and a terminal area for inputting/outputting an electric signal into/from the electronic part, wherein an electrical connection is made between the terminal areas of said two substrates by use of a flexible substrate having a structure constituted of at least three layers, and thereby a high frequency signal is transmitted between the electronic part mounted on one substrate and the electronic part mounted on the other substrate through the flexible substrate while confining an electromagnetic field. In addition, the present invention provides an optical transmission module characterized in that a slit formed in a ground plane on a connection section of a three-layer flexible substrate reduces a disturbance in electromagnetic field, making it possible to improve the transmission characteristic of a high frequency signal.