To obtain a slender bubbleless glass material by laminating a glass soot onto a substrate, sintering a porous part into a dense glass, drawing the resultant glass, affording a glass cane and heat-treating the formed glass cane at a high temperature in a short time.

A glass cane 14 is uniformly heated over the total length in a furnace 16 in a control led time to reduce bubbles. The diameter of the cane 14 is preferably 5-10mm. The figure shows a core cane 30 during the heat treatment in a high-temperature region 44 in the furnace 42 and the core cane 30 is heated in the interior of the furnace 42. The high-temperature region 44 is achieved with a heating means 45 and maintained at 1,490-1,495°C. A chuck 46 is capable of holding one end of the core cane 30 and a shaft 48 is capable of connecting the chuck 46 to a rotating motor. The core cane 30 is driven at a constant speed, preferably about 6mm/min back and forth in the interior of the high-temperature region 44. The core cane 30 is rotated at about 3.5 revolutions/min during the crossing a muffle.