PURPOSE: To obtain the title continuous preform having low loss free from deformation and crystallization by heating by reacting an activated fluorine gas with a volatile raw material.

CONSTITUTION: A chamber 1 having a fluorine gas-containing gas inlet 2 and a volatile raw material inlet 3 is evacuated and heated by a heater 4 surrounding the circumference. Then a volatile raw material charged into an evaporator 6 is heated by a heater 8 and the evaporated raw material is introduced through a feed pipe 7 thermally insulated by a heat retaining heater 9 into the chamber 1. Simultaneously a fluorine gas excited into an active state in the fluorine gas-containing activating device 10 is introduced through a feed pipe 11 to the chamber 1, reacted with the volatile raw material and deposited on a substrate 5 arranged in the central part of the chamber 1.