A splicer and a method for fusion-splicing optical fibers are provided, in which the disconnection of a fusion spliced portion of optical fibers can be prevented when the windbreak cover is opened upon fusion splicing. A method for fusion-splicing optical fibers includes: holding optical fibers on optical fiber holders which are arranged on a pair of movable stages, respectively and covering the holders with a windbreak cover; and butting the optical fibers each other by bringing the pair of movable stages mutually closer; fusion-splicing the butted optical fibers together; and loosening, before opening the windbreak cover 4, the tension applied to optical fibers between the fiber holding parts and positions at which the windbreak cover contacts with the optical fibers.