PURPOSE:To obtain the titled apparatus, having a feeding means for distillation column waste water discharged in distilling crude methanol and feeding means for air, filled with packings therein and capable of decomposing formic acid an in the above-mentioned waste water with oxygen and preventing corrosion of humidifier constituent materials. CONSTITUTION:Waste water from a distillation column discharged in distilling crude methanol in the distillation column is utilized as makeup water for a humidifier used in humidifying natural gas for steam reforming in a methanol synthesis plant. In the process, an apparatus 1 for producing the above-mentioned makeup water, having a feeding means 5 for the afore-mentioned waste waster and a feeding means 6 for air and filled with packings, preferably a catalyst material 2 for decomposing formic acid therein is used to oxidatively decompose and remove strong corrosive formic acid present in the above-mentioned waste water as a product of side reaction of methanol synthesis with oxygen in in expensive air. Thereby corrosion of humidifier constituent materials by the formic acid in the afore-mentioned waste water is reduced.