A positive chemical amplified photoresist composition having as a matrix resin a polymer having the repeating unit of Formula (I) and a photoacid generator. The polymer ranges, in polystyrene-reduced weight average molecular weight, from about 2,000 to 1,000,000. The photoresist composition is possible to develop in alkali and shows excellent sensitivity, resolution and transmissivity to deep uv light in addition to being superior in storage preservativity. The repeating unit of Formula (I) is: ##STR1## wherein, R.sub.2, R.sub.2 and R.sub.3 are independently represented by a hydrogen atom or a methyl group; R.sub.4 is a hydrogen atom, an alkyl group or an alkoxy group; R.sub.5 is functions as an acid-labile protective group and is selected from a t-butyl group, a tetrahydropyranyl group or an alkoxymethylene group; j is an integer of 1-8; k is an integer of 0-8; and 1, m and n each represent a mole ratio, satisfying the condition of l+m+n=1 where 0