To provide a highly expressed F3′5′H gene, whereby a blue moth orchid can be produced and a method using the gene for producing Orchids having blue flowers.A gene encoding a flavonoid 3′,5′-hydroxylase of Commelina communis, which has an amino acid sequence depicted in SEQ ID No: 2 or an amino acid sequence having at least 90% of homology to the amino acid sequence depicted in SEQ ID No: 2. A method for producing an orchid having a blue flower, which comprises transfecting a white moth orchid with the gene, a gene encoding a dihydroflavonol 4-reductase of Torenia or Gerbera; a gene encoding a flavanone 3-hydroxylase; and a gene encoding an anthocyanidin synthase and expressing the genes.