PURPOSE: To obtain the title resin having a high fluorine content by conducting fluorination under a mild condition in a short time efficiently and uniformly, by drawing a polyolefin resin before treating it with fluorine.

CONSTITUTION: A polyolefin resin (e.g., polyethylene resin) is molded into a sheet etc., and is drawn by using a drawing machine. The drawing ratio is pref. 3 or more. The drawn resin is placed in a reactor, which is deaerated to a high vacuum and wherein furthermore an inert gas such as nitrogen gas is substd. for the residual gas and a gas contg. fluorine is introduced to carry out fluorination to thereby obtain a fluorinated polyolefin resin. The obtd. resin contains at least 1.5 times as much bound fluorine as that of the case where an undrawn resin is fluorinated.