To obtain a positive photoresist composition which gives a rectangular photoresist having high sensitivity and resolution by forming an alkali-soluble polysiloxane having specific structural units.

The alkali-soluble polysiloxane has a wt. average mol.wt. of 500-200,000 and contains 10-100 mol% structural units represented by formula I. The photoresist composition contains (A) a polysiloxane containing an alkali- soluble polysiloxane, (B) 0.001 to 40 wt.% compound which, when exposed to light, is decomposed and generates an acid (e.g. an oxazole derivative or an s-triazine derivative), and (C) 5-50 wt.% phenolic compound having part of the phenolic hydroxy groups protected by acid-decomposable groups. In the formula, n is 1-6; L is -A-OCO-, -A-NHCO-, -A-CONH- or the like; A is a single bond or arylene; X is a single bond or a divalent linking group; Z is a group represented by formula II or III; Y is H, an alkyl, aryl or the like; i is 1-3; and m is 1-3.