An olefin polymerization reactor according to the present invention comprises: a vertically extending cylinder; a decreasing diameter portion on the cylinder, having an inside diameter that decreases progressively downward, and having a gas inlet orifice at a bottom end thereof; and a plurality of through holes passing through from an outside surface towards an inside surface of the decreasing diameter portion. Inside a reaction zone enclosed by an inside surface of the decreasing diameter portion and an inside surface above the decreasing diameter portion of the cylinder, a spouted-fluidized bed or a spouted bed is formed.