In the presence of a metal catalyst such as a samarium compound, an oxime ester compound shown by the formula (1): ##STR1## wherein R.sup.1, R.sup.2, and R.sup.3 are the same or different from each other, and each represents a non-reactive atom or a non-reactive organic group; and R.sup.2 and R.sup.3, together with the adjacent carbon atom, may bond together to form a ringis reacted with a cyanogenation agent such as an .alpha.-cyanohydrin compound (e.g., acetone cyanohydrin)to form an .alpha.-aminonitrile derivative. By hydrolyzing the .alpha.-aminonitrile derivative, the corresponding .alpha.-amino acid or a salt thereof can be obtained. According to the above processes, an .alpha.-aminonitrile derivative and an .alpha.-amino acid can be obtained in high yields.