A photoacid generator which can generate an acid efficiently when energy was absorbed, is excellent in the developing property and can form fine patterns, and a cationic polymerization initiator excellent in curability are provided; and a resist composition and a cationically polymerizable composition using them are provided.An aromatic sulfonium salt compound represented by the General Formula (I) below: (wherein each of E1 to E4 independently represents a substituent represented by the General Formula (II) below or the General Formula (III) below). Preferably, in the General Formula (I), r and s are 0; m and n are 0; or n and r are 0, and more preferably, one of m and s in the General Formula (I) is 1.