There is provided a process for preparing a fluorine-containing polymer for resist which is excellent in transparency in a vacuum ultraviolet region, comprises a structural unit derived from a fluorine-containing ethylenic monomer and/or a structural unit derived from a monomer which can provide an aliphatic ring structure in the polymer trunk chain and may have a fluorine atom, and has an acid-reactive group Y1 reacting with an acid or a group Y2 which can be converted to the acid-reactive group Y1, in which the fluorine-containing ethylenic monomer and/or the monomer which can provide an aliphatic ring structure in the polymer trunk chain are subjected to radical polymerization by using an organic peroxide represented by the formula (1): wherein R50 and R51 are the same or different and each is a hydrocarbon group having 1 to 30 carbon atoms which may have ether bond (an atom at an end of bond is not oxygen atom); p1 and p2 are the same or different and each is 0 or 1; p3 is 1 or 2, and also there is provided a photoresist composition comprising the obtained polymer. The fluorine-containing polymer is excellent in transparency in a vacuum ultraviolet region, and can form an ultra fine pattern as a polymer for a photoresist, particularly for a F2 resist.