Optical signal transmission system comprising optical signal transmission board having first optical waveguide, and optical connection board having second optical waveguide and inserted into opening of optical signal transmission board substantially perpendicularly thereto, opening being provided in upper surface of transmission board. The board comprises: a first board; the first optical waveguide extended on an upper surface of the first board; and a second board made parallel to the first board, the opening extended from the upper surface thereof toward the first board is provided in the optical signal transmission board. The optical connection board comprises: a third board; second optical waveguide extended on an upper surface of the third board; and reflection surface provided in end portion of second optical waveguide, reflecting light traveling through the second optical waveguide, and making the light incident onto the first optical waveguide extended in a direction substantially perpendicular to the board.