To provide a sulfurous acid-based water treatment agent capable of adding desirable anticorrosion, preventing oxidation of a sulfite ion and having excellent storage stability.SOLUTION: There is provided a sulfurous acid-based water treatment agent to be added to a boiler feed water, containing sulfurous acid, silicic acid or silicate, and ethylene diamine tetraacetic acid or a salt thereof, and having content of the sulfurous acid of 10 to 25 mass%, content of silicon dioxide derived from the silicic acid or the silicate of 1 to 15 mass% and concentration of the ethylene diamine tetraacetic acid or the salt thereof of 0.01 to 5 mass%.SELECTED DRAWING: None
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