the method is characterized by the surface treatment technique in which the surfaces of a core rod and a cladding tube are treated with a mixture of specific gas and oxygen under high temperature after insertion of the core rod into the cladding tube. this technique has enabled elimination of scattering loss caused by interface imperfections which is inevitably associated with the conventional rod-in-tube method. the advantages of the method are as follows' 1) an excellent reproducibility of low loss and precise dimensions of the fiber is realized in the successive productions 2) a large size preform can be made and the manufacturing process is simple, therefore mass productivity is very high. 3) due to wider freedom in determining fiber construction, fibers ranging from a small core size for single mode transmission to a large core size suitable for power transmission can be made. an outline of the method and characteristics of fibers prepared by the method are reported