A light by-passing device is connected in cascade between transparent cylindrical optical conductors, and is used for the purpose of by-passing and taking out a part of the light transmitted through them. Said light by-passing device is comprised of first and second transparent cylindrical members and three spacers of the same thickness interposed between the first and second cylindrical members. The shape of the first and second cylindrical members' edge surfaces are opposed to each other through the spacers that have first horizontal water level surfaces, inclined surfaces following the first horizontal surfaces, and second horizontal water level surfaces following the inclined surfaces. The spacer interposed between the inclined surfaces has a round hole through which light passes. It also has a grasping part used to insert or take out.