PURPOSE: To obtain a base material having low absorption loss due to OH ion by inserting a quartz glass rod into a quartz glass tube after removing the surface layer of the quartz glass rod with HF soln. and modifying the parts of the rod and the tube to be bonded by fusion by heating under specified condition to unite the rod and the tube to form one body.

CONSTITUTION: A quartz glass rod prepd. by the vapor epitaxial deposition process is treated with 20W50wt% HF to remove its surface layer by 30W150μ. The treated rod is inserted into a quartz glass tube, and a gaseous mixture consisting of 100pts.vol. O2, 0.5W100pts.vol. surface treating agent (compd. contg. element for forming oxides of glass), and ≤0.1wt% H2, is fed to the space between the tube and the rod. The external peripheral surface of the tube is heated uniformly at 1,200W1,400°C to modify the parts to be bonded by fusion of the tube and the rod. Further, the tube is heated at 1,900W2,300°C to bond the rod to the tube tightly by fusion to form one body. Thus, a base material for optical fiber is obtd.