A polycrystalline silicon reactor which can prevent polycrystalline silicon which deposits on the surface of an electrode holding a silicon seed rod from being peeled off is provided. In a polycrystalline silicon reactor which applies an electric current to a silicon seed rod provided within a furnace, thereby heating the silicon seed rod, brings a source gas supplied into the furnace into reaction, and deposits polycrystalline silicon on the surface of the silicon seed rod, the reactor includes, at a bottom plate of the furnace, an electrode holder provided so as to be electrically insulated from the bottom plate, and a seed rod holding electrode connected to the electrode holder, and holding the silicon seed rod toward the upside. Concavo-convex portions exposed to a furnace atmosphere is provided at an outer peripheral surface of the seed rod holding electrode.