To provide a corrosion prevention technique where the material cost of the throat member of a prescrubber is reduced and high corrosion resistance is kept.

Corrosion at the throat part 4 of a prescrubber is suppressed by the oxidation of SO₂ or a sulfurous acid contained in a liquid to SO₄^2- due to the addition of an oxidant to a prescrubber cleaning liquid consisting of a limestone slurry and then significant cost reduction can be achieved. It is also possible that obtaining the necessary oxidant and pH adjustment are performed by the electrolysis of sea water.