An olefin polymerization reactor comprising a tubular portion that extends in a vertical direction; a tapered portion disposed inside the tubular portion, an inside diameter of the tapered portion decreasing downwardly, and including a vertically penetrating orifice at a bottom end thereof; a tubular lower deflector disposed above the orifice of the tapered portion, an outside diameter of the tubular lower deflector increasing downwardly, a bottom end thereof being located away from an inner wall of the tubular portion, and including a vertically penetrating orifice at a top end thereof; and an upper deflector disposed above the orifice of the lower deflector, an outside diameter of the upper deflector increasing downwardly, a top end thereof being closed, a bottom end thereof being located away from the inner wall of the tubular portion, and an outside diameter at the bottom end being equal to or greater than an inside diameter of the orifice of the lower deflector.