The present invention provides a sulfonium compound represented by the formula (I): wherein Q1 and Q2 each independently represent a fluorine atom or a C1-C6 perfluoroalkyl group, R1 represents a C5-C42 organic group having a β-ketoester structure and A+ represents an organic counter ion, and a chemically amplified photoresist composition comprising the above-mentioned sulfonium compound and a resin comprising a structural unit having an acid-labile group and being insoluble or poorly soluble in an aqueous alkali solution but becoming soluble in an aqueous alkali solution by the action of an acid.