A method for increasing the amount of trans-1,4-polyisoprene contained in a plant, and a method for effectively producing trans-1,4-polyisoprene using a plant are provided. A long-chain trans-prenyl diphosphate synthase gene that comprises DNA having at least one base sequence selected from the group consisting of a base sequence from positions 88 to 1134 of the base sequence of SEQ ID NO: 1 or a complementary sequence thereof, a base sequence from positions 42 to 1088 of the base sequence of SEQ ID NO: 3 or a complementary sequence thereof, and a base sequence from positions 91 to 1140 of the base sequence of SEQ ID NO: 5 or a complementary sequence thereof are disclosed; as well as a plant transformed with an expression vector containing the gene.