To easily and rapidly splice optical fibers of different types with low light power loss by automatically selecting the threshold value for matching mode field diameter.

The matching of the mode field diameters of the optical fibers is completed by extension heating (step 57) after the execution of splicing (step 55). The hot fiber indices at the two fiber ends are continuously determined during extension heating. One of the indices or the quantity led out thereof is compared with the threshold value at all times and the heating is stopped when one of the indices or the quantity led out thereof attains the threshold value. The threshold value is determined by using test fiber pieces of the same kind in the preceding stage in the splicing action during heating. Namely, the corresponding hot fiber indices when the transmission of the incessantly measured light has the max. value are stored in addition to the indices and the threshold value is derived therefrom.