An optical fiber splicing unit includes: a mechanical splice which aligns optical fibers and puts the optical fibers between half-split elements to splice the optical fibers to each other; a splice holder portion which holds the mechanical splice; fixing member guide portions which respectively guide anchoring fixation members that are respectively fixed to the optical fibers at two sides of the held mechanical splice; anchoring portions which respectively anchor the anchoring fixation members, the anchoring fixation members being respectively guided by the fixing member guide portions and advancing; and a first spacer which abuts one anchoring fixation member and is disposed to be retracted to restrict the advancement of said one anchoring fixation member, said one anchoring fixation member being guided by one fixing member guide portion and advancing.