A positive chemical amplified photoresist composition, comprising as a matrix resin a polymer with a polystyrene-reduced weight average molecular weight of 2,000 to 1,000,000, having the repeating unit represented by the following formula (I):
- ##STR9## wherein R.sub.1, R.sub.2 and R.sub.3 are independently selected from the group consisting of a hydrogen atom and a methyl group;
- R.sub.4 is selected from the group consisting of a hydrogen atom, an alkyl group and an alkoxy group;
- R.sub.5 functions as an acid-labile protective group and is selected from the group consisting of a t-butyl group, a tetrahydropyranyl group and an alkoxymethylene group;
- j is an integer of 1-8;
- k is an integer of 0-8;
- and l, m and n each represent a mole ratio satisfying the condition of "l+m+n=1" where 0<1<0.4;
- and a photoacid generator.