An optical fiber fusion splicer for splicing two optical fibers having different mode field diameters, comprising a heating source, a first moving block on which a first aligning arm for locating and fixing a first optical fiber having a mode field diameter is positioned, and a second moving block on which a second aligning arm for locating and fixing a second optical fiber, which has a mode field diameter differing from a mode field diameter of said first optical fiber, and which is spliced and fusion connected with said first optical fiber, is positioned,
- wherein said first moving block is mounted on the second moving block so as to move in a longitudinal direction of said optical fibers.