Abstract: An aqueous formaldehyde solution having a formaldehyde concentration of 37 to 60% by weight is produced with less consumption of methanol under methanol-rich mild reaction conditions from a feed gas containing methanol, air, and steam, and, if necessary, an inert gas by subjecting the feed gas to catalytic reaction over a silver catalyst, contacting the resulting product gas with absorbing water, thereby absorbing the formaldehyde into the absorbing water from the product gas in an absorption column, and recovering the desired aqueous formaldehyde solution as bottoms of the absorption column, wherein a total of a corresponding amount of water to that of steam to be added to the feed gas and a necessary amount of water for adjusting the concentration of the product aqueous formaldehyde solution is added to the top of the absorption column as the absorbing water, and an aqueous solution containing 0.1 to 3.0% by weight of methanol and 0.1 to 1.