Disclosed is a precursor composition comprising: a compound selected from a compound represented by the formula: Si(OR1)4 and a compound represented by the formula Ra(Si)(OR2)4−a (in the formulas R1 represents a monovalent organic group; R represents a hydrogen atom, a fluorine atom or a monovalent organic group; R2 represents a monovalent organic group; and a is an integer ranging from 1 to 3, provided that R, R1 and R2 may be the same or different from one another) a thermally degradable organic compound; an element having a catalyst activity; urea; and the like. A porous thin film produced from the precursor composition is irradiated with ultraviolet ray, and then subjected to gas-phase reaction with a hydrophobic compound. A porous thin film thus prepared can be used for the manufacture of a semiconductor device.