The present invention provides a protein having the amino acid sequence as shown in SEQ ID NO: 2, 4, 6, 8, 10 or 12 or a protein having a modified amino acid sequence thereof and having an activity of transferring an aromatic acyl group to a sugar residue of a flavonoid; a gene, especially cDNA, encoding the protein; and use thereof. For example, by introducing the above gene into a plant expressing hydroxycinnamate 1-O-glucosyltransferase gene, optionally together with a cDNA encoding a protein having the amino acid sequence as shown in SEQ ID NO: 14, 16 or 18 or a protein having an amino acid sequence derived therefrom by modification and having an activity of glucosylating a hydroxyl group at position 1 of hydroxycinnamic acid, and then expressing the introduced gene(s), it is possible to acylate the sugar residue of flavonoids in flowers of the plant to thereby confer a blue color on the flowers.