An olefin polymer having Mw/Mn (Mw: weight-average molecular weight, Mn: number-average molecular weight) of less than 2.00, which is a multiblock copolymer comprising four or more polymer blocks comprising (i) a polymer block (a) obtained from ethylene, (ii) a polymer block (b) obtained from one α-olefin selected from α-olefins of 2 to 20 carbon atoms or a polymer block (c) obtained from two or more α-olefins selected from α-olefins of 2 to 20 carbon atoms, and plural polymer blocks (iii) each being obtained from one α-olefin selected from α-olefins of 2 to 20 carbon atoms or being a random copolymer block obtained from two or more α-olefins selected from α-olefins of 2 to 20 carbon atoms, and wherein the polymer block (a) is selected from a polyethylene, a high-density polyethylene, and a linear low-density polyethylene, adjacent polymer blocks of the four or more polymer blocks being different from each other; wherein the weight-average molecular weight, the number-average molecular weight and Mw/Mn (Mw: weight-average molecular weight, Mn: number-average molecular weight) are measured in orthodichlorobenzene solvent at 140°C by means of GPC (gel permeation chromatography) and converted to a molecular weight in terms of polystyrene by a universal method, wherein, if the polymer is a monodisperse ethylene/propylene copolymer, the molecular weight is calculated in terms of polyethylene, and when the polymer is another polymer, the molecular weight is calculated in terms of polypropylene., using the following parameters Polystyrene standard sample: K=0.000137, α=0.686 Polyethylene: K=0.000326, α=0.77 Polypropylene: K=0.0001, α=0.8. number-average molecular weight) of less than 2.00, which is a multiblock copolymer comprising four or more polymer blocks comprising (i) a polymer block (a) obtained from ethylene, (ii) a polymer block (b) obtained from one α-olefin selected from α-olefins of 2 to 20 carbon atoms or a polymer block (c) obtained from two or more α-olefins selected from α-olefins of 2 to 20 carbon atoms, and (i) a polymer block (a) obtained from ethylene, (ii) a polymer block (b) obtained from one α-olefin selected from α-olefins of 2 to 20 carbon atoms or a polymer block (c) obtained from two or more α-olefins selected from α-olefins of 2 to 20 carbon atoms, and plural polymer blocks (iii) each being obtained from one α-olefin selected from α-olefins of 2 to 20 carbon atoms or being a random copolymer block obtained from two or more α-olefins selected from α-olefins of 2 to 20 carbon atoms, and wherein the polymer block (a) is selected from a polyethylene, a high-density polyethylene, and a linear low-density polyethylene, adjacent polymer blocks of the four or more polymer blocks being different from each other; wherein the weight-average molecular weight, the number-average molecular weight and Mw/Mn (Mw: weight-average molecular weight, Mn: number-average molecular weight) are measured in orthodichlorobenzene solvent at 140°C by means of GPC (gel permeation chromatography) and converted to a molecular weight in terms of polystyrene by a universal method, wherein, if the polymer is a monodisperse ethylene/propylene copolymer, the molecular weight is calculated in terms of polyethylene, and when the polymer is another polymer, the molecular weight is calculated in terms of polypropylene., using the following parameters Polystyrene standard sample: K=0.000137, α=0.686 Polyethylene: K=0.000326, α=0.77 Polypropylene: K=0.0001, α=0.8.