The invention equalizes the lengths of optical paths required in optical communication among optical paths constructed by plural optical fibers. Optical fibers parallel to each other at non-equal intervals are set to ADD, OUT, IN and DROP, and are used for add-drop. An ADD-OUT optical path of an optical path length Anull2E from the optical fiber to the optical fiber through a fixing mirror and a movable mirror, and an IN-DROP optical path of an optical path length Cnull2E from the optical fiber to the optical fiber through a movable mirror and a fixing mirror are formed. When the movable mirrors are escaped, an ADD-DROP optical path of an optical path length AnullBnullCnull2E from the optical fiber to the optical fiber through the fixing mirrors, and an IN-OUT optical path of an optical path length Bnull2Dnull2E from the optical fiber to the optical fiber through fixing mirrors are formed. Three optical paths except for ADD-DROP are set to the same length by setting AnullCnullBnull2D.