In a process for hydrating a nitrile compound by the action of nittrilase produced by a microorganism having nitrilase activity to convert the nitrile compound into the corresponding amide compound, the performances such as yield and reaction velocity are markedly enhanced by irradiation of the nitrilase produced by the microorganism with light. The process of the present invention is characterized in that (a) the nitrilase produced by the microorganism is in the form of crushed cells of the microorganism or intra-cellular substance obtained from the microorganism, (b) the nitrilase produced by the microorganism is allowed to accept light energy of at least about 10 µE/g microbial cells 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.