PURPOSE: To obtain an excellent deoiling effect without inducing deterioration of strength of a continuously running widen long glass cloth by dipping in hot water and subsequently uniformly blowing a high-speed fine diameter liquid flow to the surface of said glass cloth.

CONSTITUTION: A widen long cloth is continuously transferred and subjected to hot water-dipping treatment to make a treating agent such as starch or oils and fats attached to the surface of glass filament constituting said cloth to swollen state, then subsequently a high-speed fine diameter liquid flow is uniformly blown to the surface of said glass cloth by a high-pressure liquid jetting device, thus said swollen treating agent attached to the surface of said cloth is released and removed with separating and opening said constituting filament by impact force of said liquid.