A method for manufacturing an optical fiber preform, comprising: synthesizing glass particles including a dopant using a plasma flame generated by a plasma torch; consecutively moving a glass rod forward and backward relative to the plasma flame while rotating; depositing the glass particles on the glass rod by blowing the glass rod with the plasma flame while the glass rod moves forward relative to the plasma torch and rotates, the plasma torch being supplied with glass raw material and a fluorine source, preferably silicon tetrachloride and silicon tetrafluoride, together with argon and oxygen; setting the power supplied to the plasma torch to a minimum power needed to safely maintain the plasma and only supplying argon and oxygen without the said glass raw material and without the said fluorine source to the plasma torch while the glass rod moves backward relative to the plasma torch and rotates; and cooling the glass rod heated by the plasma flame using at least two forced cooling nozzles, wherein the depositing is performed during the cooling, the at least two forced cooling nozzles and an exhaust outlet are provided on the opposite side of the glass rod as the plasma torch and at positions opposite the upper and lower edges of the plasma torch, i.e. near the upper and lower edges of the exhaust outlet provided across from the plasma torch, and the cooling includes blowing the glass rod with a gas having a temperature that is lower than the glass rod heated by the plasma flame, wherein each forced cooling nozzle blows room-temperature air to cool the upper and lower edges of the portion undergoing deposition. synthesizing glass particles including a dopant using a plasma flame generated by a plasma torch; consecutively moving a glass rod forward and backward relative to the plasma flame while rotating; depositing the glass particles on the glass rod by blowing the glass rod with the plasma flame while the glass rod moves forward relative to the plasma torch and rotates, the plasma torch being supplied with glass raw material and a fluorine source, preferably silicon tetrachloride and silicon tetrafluoride, together with argon and oxygen; setting the power supplied to the plasma torch to a minimum power needed to safely maintain the plasma and only supplying argon and oxygen without the said glass raw material and without the said fluorine source to the plasma torch while the glass rod moves backward relative to the plasma torch and rotates; and cooling the glass rod heated by the plasma flame using at least two forced cooling nozzles, wherein the depositing is performed during the cooling, the at least two forced cooling nozzles and an exhaust outlet are provided on the opposite side of the glass rod as the plasma torch and at positions opposite the upper and lower edges of the plasma torch, i.e. near the upper and lower edges of the exhaust outlet provided across from the plasma torch, and the cooling includes blowing the glass rod with a gas having a temperature that is lower than the glass rod heated by the plasma flame, wherein each forced cooling nozzle blows room-temperature air to cool the upper and lower edges of the portion undergoing deposition.