A river water utilizing flue gas desulfurization system 10 according to an embodiment of the present invention includes: a pre-treatment facility 13 that removes humic substances in river water 11 to produce makeup water 12; and a desulfurization apparatus 17 that brings sulfur content in flue gas 15 into contact with limestone gypsum slurry 16 in an apparatus body 14. The pre-treatment facility 13 includes a flocculant mixing basin 22 in which a flocculant 21 is added to the river water 11 to flocculate and remove the humic substances contained in the river water 11; and an activated carbon absorption unit 23 in which the humic substances contained in the river water 11 are absorbed and removed by way of activated carbon.