Raffinose synthase genes coding for proteins capable of producing raffinose by combining a D-galactosyl group through an .alpha.(1.fwdarw.6) bond with a hydroxyl group attached to the carbon atom at position 6 of a D-glucose residue in a sucrose molecule were isolated from various plants. These raffinose synthase genes are useful to change the content of raffinose family oligosaccharides in plants.