To manufacture a polyolefin by recycling an unreacted monomer with an excellent efficiency and at a low cost.

This polymerization apparatus 1 includes: a liquid-phase polymerization reactor 2 for obtaining a slurry product by the liquid-phase polymerization of an olefin monomer; a gas-phase polymerization reactor 3 for gas-phase polymerizing the above slurry product; and a condenser 4 for condensing and liquefying an unreacted olefin monomer in the above gas-phase polymerization reactor 3 and additionally includes: a liquefied monomer transfer line 11 for feeding a liquid product in the above condenser 4 into the polymerization apparatus 1 from the condenser 4. Thus, the capability of feeding again the unreacted monomer recovered directly to the liquid-phase polymerization without passing through such a treatment as purification or the like, permits the unreacted olefin monomer to be recycled with an excellent efficiency and at a low cost.