To obtain a positive type resist composition having high sensitivity and high resolving power, giving a rectangular photoresist, having good wettability with a developing solution, nearly free from development defects and ensuring a slight dimensional shift in pattern transfer to a lower layer in an oxygen plasma etching step.

The positive type photoresist composition uses an acid decomposable polymer containing repeating units of formula I [where M1 is an atomic group for forming an alicyclic structure; (n) is 1 or 2; L is a single bond or a (n+1)-valent combining group; and R', R" and R''' are each an alkyl, phenyl, trialkylsilyl or trialkylsilyloxy] and repeating units of formula IIa and/or formula IIb [where Z is -O- or -N(R3)-; X1 and X2 are each -O-, -S-, -NH- or -NHSO2-, A1 and A2 are each a single bond or a divalent combining groupt and R1 and R2 are each H, cyano, hydroxyl, -COOH, -COOR5, -CO-NH-R6, alkyl, alkoxy or a cyclic hydrocarbon].