PURPOSE: To equalize load, to lengthen a leakage path, to reduce the possibility of a leakage after a fusion and to improve quality by fusion-bonding the cut surface section of a thick glass pipe and a discoid glass surface plate by an adhesive glass tape under the state in which load is applied to the discoid glass surface plate.

CONSTITUTION: An adhesive glass tape 12 is contact-bonded with the plane of a thick glass tube 11, which forms a Brewster angle and is polished optically. A discoid glass surface plate 13 is arranged on the tape 12, and fusion-bonded under the state in which load is applied. The inner diameter of the thick glass tube 11 is reduced naturally because of a thick glass plate, a contact area with the thick glass tube 11 through the adhesive glass tape 12 of the discoid glass sur face plate 13 is widened, and a leakage path is lengthened remarkably. It is preferable that the diameter of the discoid glass surface plate is larger than the maximum diameter of the inner diameter of the cut edge of a cut surface at the Brewster angle of the thick glass tube 11 and is smaller than the minimum diameter of the outer diameter of the cut edge at that time. Accordingly, a leakage after a fusion resulting from the deviation of load can be prevented.