PURPOSE: To obtain a preform rod suitable for the manufacture of an optical fiber having low light transmission loss, by carrying out the dehyration treatment of a porous preform controlling the temperature, the rate of dehydration and the flow rate of chlorine gas within specific ranges.

CONSTITUTION: A raw material to glass, e.g. SiCl4, is supplied to an oxyhydrogen burner, and the soot is synthesized by the hydrolysis in the flame and at the same time, the soot is grown along the axial direction of the rotating quartz rod target 1 to obtain a porous preform 2. The porous preform 2 is heated with the heater 3a of the surrounding electrical furnace 3 at a dehydration temperature, i.e. at the surface temperature of 850W950°C, e.g. 900°C. Chlorine gas is supplied to the electrical furnace 3a at a rate of 50W100cc/min, e.g. 70cc/min during the dehydration treatment, and the porous preform 2 is pulled up at a rate of 2.5W3.5mm/min, e.g. 3mm/min by pulling up the quartz rod 1. The porous preform 2 can be dehydrated by this process.