It is intended to provide by improving a known method of producing hyaluronic acid in a plant, a plant or a cultured plant cells which can produce hyaluronic acid at a lower cost and a further higher yield than before, a method of preparing the same, an expression vector for transformation, a method of producing hyaluronic acid using the plant or the cultured plant cells and the like. The method of producing hyaluronic acid comprising obtaining hyaluronic acid by co-expressing a protein with hyaluronic acid synthase activity and an exogenous protein with sugar-nucleotide synthase activity in a plant cell or a plant is provided.