In a process for hydrating and hydrolyzing a nitrile compound by the action of a microorganism having nitrilase activity to convert the nitrile compound into the corresponding organic acid, the performances such as yield and reaction velocity are markedly enhanced by irradiation of the microorganism with light. The process of the present invention is characterized in that (a) the microorganism having nitrilase activity is a positive Gram-staining microorganism, (b) the microbial cells are allowed to accept light energy of at least about 2 x 10 3 µE/g microbial cells.second before termination of the hydration reaction, and (c) the hydration reaction is carried out in a vessel composed at least partly of a non-light-transmitting material.