A process for producing a highly stable solution of monopersulfuric acid, characterized by including mixing hydrogen peroxide of 35 wt.% or higher with sulfuric acid of 70 wt.% or higher, reacting them, cooling the reaction solution to 80°C or below within 5 min from the initiation of the mixing and thereafter diluting the reaction solution with water amounting to at least 4 times the weight thereof. Further, there is provided a monopersulfuric acid continuous production apparatus characterized by having a first static mixer for mixing and reaction of hydrogen peroxide and sulfuric acid； a second static mixer for mixing the reaction solution obtained by the first static mixer with the diluent water； and a reaction solution transfer pipe fr...
