A monosulfonium salt in which very little unreacted raw material remains, which has a purity of at least 96%, and which has one sulfonio group in its molecule is manufactured without a refining step. After (a) an aryl compound, (b) a sulfoxide compound, (c) a dehydrating agent, and (d) a BF4, PF6, AsF6, or SbF6 salt of an alkali metal or an alkaline earth metal are introduced into a reaction system, (e) an inorganic acid is added, so that the aryl compound (a) and the sulfoxide compound (b) are subjected to dehydration condensation.