NOVELTY - The fluidized bed reactor comprises a reaction pipe comprising silicon particles, a unit to supply reaction gas comprising silicon elements to an internal space of the reaction pipe along a reaction gas channel, a heater unit that is configured to supply heat to the internal space of the reaction pipe and provided with a heater channel, and a plate configured to fix the reaction gas supply unit and the heater unit. An inert gas flows in the reaction gas channel. The channel comprises a first channel, and a second channel connected with the first channel.    USE - The fluidized bed reactor is useful for producing a polycrystal silicon, which is useful for a semiconductor device or a solar cell.    ADVANTAGE - The fluidized bed reactor is capable of easily and efficiently producing the polycrystal silicon without any contamination, and can be stably operated for long time without degrading its performance.    DETAILED DESCRIPTION - The fluidized bed reactor comprises a reaction pipe comprising silicon particles, a unit to supply reaction gas comprising silicon elements to an internal space of the reaction pipe along a reaction gas channel, a heater unit that is configured to supply heat to the internal space of the reaction pipe and provided with a heater channel, and a plate configured to fix the reaction gas supply unit and the heater unit. An inert gas flows in the reaction gas channel. The channel comprises a first channel, and a second channel connected with the first channel, where a passage direction of the gas flowing in the first channel is opposite to a passage direction of the gas flowing in the second channel. The plate comprises a connection channel to make the channel of the reaction gas supply unit in communicate with the heater channel of the heater unit. A temperature of the reaction gas supply unit is maintained to be lower than a silicon deposition temperature inside the reaction pipe. The inert gas is supplied to an entire rim of the reaction gas supply unit. The heater unit comprises a heater, and a heater cap configured to cover the heater. The heater cap is formed of quartz. A supply pressure of reaction gas is maintained uniformly in the reaction gas channel. The connection channel comprises a first connection channel formed from a center of the reaction gas supply unit in a radial direction, and a second connection channel crossing with the first connection channel. The first and second connection channels are spatially close to each other.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a fluidized bed reactor.    Head (100)    Lining layer (150)    First body part (200)    First reaction pipe (250)    Second body part. (300)