The present invention provides a method for producing α-olefin polymer, comprising polymerizing α-olefins in the presence of a halogen atom-containing acid catalyst by use of an aliphatic hydrocarbon having 3-12 carbon atoms, which contains 5 to 95 mass % of α-olefin, as a raw material; removing catalyst residue and halogen-containing compounds by bringing a product in the polymerization step into contact with an aluminum atom-containing inorganic solid treatment agent with or without deactivating the catalyst; and separating the α-olefin polymer. According to this method, the catalyst residue and other halogen-containing compounds can be removed, without relying on aqueous deactivation and separation steps using an alkaline aqueous solution or the like, from a polymerization reaction product obtained by polymerization in the presence of a halogen-containing acid catalyst by use of an α-olefin-containing liquid hydrocarbon as a raw material.