An object of the present invention is to provide photoacid generator, which shows very high sensitivity in the near ultraviolet range of about 300 to 400 nm, and also can remarkably increase a reaction rate of a photoreactive composition using the same, and to provide a photoreactive composition which can initiate the reaction even by irradiation with near ultraviolet light within a short time and also can obtain a desired reaction product. More particularly, the present invention provides a dithienyl sulfide disulfonium salt represented by the formula (A1): wherein R1a to R4a each independently represents an optionally substituted monocyclic carbon ring group, an optionally substituted condensed polycyclic carbon ring group or an optionally substituted monocyclic heterocyclic group, and X− represents an inorganic acid ion or an organic acid ion; a photoacid generator containing the dithienyl sulfide disulfonium salt, and a photoreactive composition containing the photoacid generator and an acid reactive compound; a dithienyl sulfide sulfonium salt represented by the formula (B1): wherein R1b and R2b each independently represents an optionally substituted monocyclic carbon ring group, an optionally substituted condensed polycyclic carbon ring group or an optionally substituted monocyclic heterocyclic group, R3b to R5b each independently represents a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 4 carbon atoms, an acyl group or a hydroxyl group, and X− represents an inorganic acid ion or an organic acid ion, a photoacid generator containing the same, and a photoreactive composition containing the photoacid generator; and a phenylthiothiophene sulfonium salt represented by the formula (C1): wherein R1c and R2c each independently represents an optionally substituted monocyclic carbon ring group, an optionally substituted condensed polycyclic carbon ring group or an optionally substituted monocyclic heterocyclic group, and R3c to R7c each independently represents a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, alkoxy group having 1 to 4 carbon atoms, an acyl group or a hydroxyl group, and X− represents an inorganic acid ion or an organic acid ion, a photoacid generator containing the same, and a photoreactive composition containing the photoacid generator and an acid reactive compound.