A reaction furnace for producing a polycrystalline silicon according to the present invention is designed so as to have an in-furnace reaction space in which a reaction space cross-sectional area ratio (S=[S0−SR]/SR) satisfies 2.5 or more, which is defined by an inner cross-sectional area (So) of a reaction furnace, which is perpendicular to a straight body portion of the reaction furnace, and a total sum (SR) of cross-sectional areas of polycrystalline silicon rods that are grown by precipitation of polycrystalline silicon, in a case where a diameter of the polycrystalline silicon rod is 140 mm or more. Such a reaction furnace has a sufficient in-furnace reaction space even when the diameter of the polycrystalline silicon rod has been expanded, and accordingly an appropriate circulation of a gas in the reaction furnace is kept.