NOVELTY - Manufacture of polycrystalline silicon involves depositing polycrystalline silicon on silicon core wire (12) by supplying source gas with chlorosilane gas and hydrogen gas. According to polycrystalline-silicon rod diameter (D), control of gas supply and silicon rod temperature is performed by conditions (A)-(C). Condition (A) includes maintenance of largest chlorosilane gas supply volume. Condition (B) includes maintenance of supply amount ratio of hydrogen gas with respect to chlorosilane gas. Conditions (C) includes silicon rod temperature fall based on silicon rod diameter expansion.    USE - Manufacture of polycrystalline silicon used as raw material of single crystal silicon for semiconductor and solar cell.    ADVANTAGE - The highly-purified polycrystalline silicon is manufactured, with suppressed generation of popcorn.    DETAILED DESCRIPTION - Manufacture of polycrystalline silicon by Siemens method involves supplying source gas containing chlorosilane gas and hydrogen gas in a reactor (100) from a nozzle mouth with relatively low gas supply volume during initial stage process (P1) and relatively high gas supply volume during later stage process (P2), and depositing a polycrystalline silicon on a silicon core wire. During intermediate process (P3), gas supply volume is raised from value of the initial stage process to value of the later process. During processes (P1)-(P3), reaction temperature is performed at 900-1250 degrees C under reaction pressure of 0.3-0.9 MPa. During later process, flow rate in nozzle mouth when carrying out gas supply by largest source gas supply amount is 150 m/second or more. According to the diameter (D) of the polycrystalline-silicon rod (13) which changes with advancing of precipitation reaction after the reaction starts, control of gas supply and silicon rod temperature is performed by following conditions (A)-(C). The condition (A) includes supply of chlorosilane gas supply in quantity of less than 1/3 of largest chlorosilane gas supply volume until supply amount of chlorosilane gas is set to D1 which is predetermined value of 15-40 mm, increase of chlorosilane gas supply volume in continuous or stepwise form until supply amount of chlorosilane gas becomes predetermined value (D2) which is more than D1 and becomes the largest chlorosilane gas supply volume, and maintenance of the largest chlorosilane gas supply volume. The condition (B) includes supply of hydrogen gas such that chlorosilane gas concentration in the source gas is 30-40 mol% until supply amount of hydrogen gas becomes D1, increase of supply amount ratio with respect to the chlorosilane gas in continuous or stepwise form, and maintenance of supply of source gas such that the chlorosilane gas concentration in the source gas is 15-30 mol% after attaining D2. The conditions (C) includes fall of temperature of silicon rod according to diameter expansion of the silicon rod, after attaining D2. An INDEPENDENT CLAIM is included for reactor used for manufacturing polycrystalline silicon by Siemens method, which has coolant circuit (3,4,6,7) for controlling surface temperature of bell-jar (1) and base plate (5), and coolant temperature control portion (16) which controls temperature of coolant (17) which flows into refrigerant circuit at 40-90 degrees C.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the reactor used for manufacturing polycrystalline silicon.    Bell-jar (1)    Coolant circuit (3,4,6,7)    Base plate (5)    Silicon core wire (12)    Polycrystalline-silicon rod (13)    Coolant temperature control portion (16)    Coolant (17)    Reactor (100)