Abstract: In a process of producing hydroxyalkanal, a raw material, namely, an unsaturated aldehyde, is hydrated with a solution in the presence of a catalyst, and alcohol is added to the solution. A adding amount of the alcohol with respect to the unsaturated aldehyde is preferably in a range between 0.001 percent by weight and 10 percent by weight. According to this process, a consecutive reaction (side reaction) of a reaction product, namely, hydroxyalkanal, is curbed by the alcohol added to the solution, thereby making it possible to produce hydroxyalkanal at high selectivity out of a high-density unsaturated aldehyde solution. Therefore, the above process can trigger a reaction of an industrially advantageous unsaturated aldehyde solution, and thus improves the yield of hydroxyalkanal.