To provide a photoacid generator having high developability, a cationic polymerization initiator having high curability, a resist composition and a cationically polymerizable composition using them.

An aromatic sulfonium salt compound represented by general formula (I) (wherein R1-R10 are each 1-18C alkyl which is unsubstituted or contains a substituent, or the like; R11-R17 are each 1-18C alkyl which is unsubstituted or contains a substituent, or the like; R18 is 1-18C alkyl which is unsubstituted or contains a substituent, or the like; and X1- is a monovalent anion) is used as a photoacid generator or a cationic polymerization initiator.