A method for producing a triarylsulfonium salt having a structure that only one of the three arobatic rings of the three aromatic rings on the cationic portion is different from the other two aromatic groups, and which is useful, for example, as an acid-generating agent for a resist or a photo cationic polymerization initiator, by reacting a diaryl sulfoxide and an aryl Grignard reagent RMgX  [2] in the presence of an activator with high affinity for oxygen of 3 to 7.5 equivalents relative to the above diaryl sulfoxide, and then reacting the resultant reaction mixture with a strong acid HA1  [3].