A method of splicing an optical fiber of the invention splices a first optical fiber cable and an optical fiber in a splicing box, the first optical fiber cable is a drop cable or an indoor cable, the optical fiber is drawn from a second optical fiber cable, the method splices a terminal of the first optical fiber cable and the optical fiber. The method includes: sliding a unit base holding an extended-optical-fiber-attached splice along a rail in a direction in which the unit base approaches a grasper; thereby inserting an inserted optical fiber grasped by the grasper between halved elements of a mechanical splice; and splicing the inserted optical fiber and an extended optical fiber by butt-jointing an end of the inserted optical fiber to the extended optical fiber.