The object of the present invention is to provide an optical fiber fusion splice method in which two optical fibers having different mode field diameters are fusion spliced with a low splice loss without complicated words and special equipment other than a fusion splice device, and an optical fiber fusion splicer which is suitably used for the optical fiber fusion method; in order to achieve the object, the present invention provides an optical fiber fusion splice method for splicing two optical fibers having different mode field diameters comprising the steps of: arranging a first optical fiber having a small mode field diameter and a second optical fiber having a large mode field diameter so that the cut surfaces thereof face each other, heating by an electric discharge and fusion splicing the cut surfaces, shifting the heating position by an electric discharge in the first optical fiber by shifting integrally the first and second optical fibers in the longitudinal direction of the optical fibers, and additional heating the first optical fiber by an electric discharge.