A die assembly includes a barrel die (4) having a tubular inner face, a sleeve (5) covering an outer peripheral surface of the barrel die, and a pair of core dies (1, 2) for pressing a preform inside the tubular inner surface of the barrel die. At least one of the paired core dies (1, 2) is slidable and guided along the tubular inner surface of the barrel die (4). The sleeve (5) does not transmit light having a wavelength of 5 µm or shorter and has a thermal conductivity of 70 W/m·K or greater.