One end side of a core wire holder 20 is formed into a shape of a truncated cone and has an inclined surface. In the end portion, an opening 22 is provided, and a hollow portion 21 is formed, a silicon core wire 5 being inserted into the hollow portion 21 and held therein. On the surface of the silicon core wire 5, polycrystalline silicon 6 is vapor deposited by the Siemens method to produce a polycrystalline silicon rod. On the inclined surface of the truncated cone portion in the vicinity of the opening 22, as a thermal insulating layer, annular slits 23a to 23c are formed from an outer circumferential surface in the vicinity of the opening toward the hollow portion 21. The annular slit acts as a thermal insulating portion, and suppresses escape of the heat to heat the one end side of the core wire holder 20.