An optical multiplexing/demultiplexing device with variable branching ratio is disclosed. The device includes a first collimator lens assembly having a first collimator lens (14), and first and second optical fibers F 1 and F 2 which have distal ends thereof optically coupled to the lens, and are away from each other by a distance "d," an optical axis of the lens being positioned at a midpoint therebetween, a second collimator lens assembly having a second collimator lens (15) and a third optical fiber F 3 which has a distal end thereof optically coupled to the lens, being away from an optical axis of the lens by a distance "d/2" and reflection mirror means. The reflection mirror means (17) reflects and couples a part or all of a parallel beam of an expanded mode field area, which is emitted from the first optical fiber and formed on a reference plane, to the second optical fiber, or transmits and connects all or a part of the parallel beam to the third optical fiber.