A method of producing an ethylene homopolymer or a copolymer of ethylene and α-olefin by solution polymerization in the presence of a catalyst system which comprises an arylphenoxy-based transition metal catalyst, and a boron compound and an organic aluminum compound as a cocatalyst, wherein pressure in a reaction system of ethylene monomers is 10 to 150 atm and a polymerization temperature is 120 to 250°C, wherein the arylphenoxy-based transition metal catalyst is expressed by Formula 1, and includes a cyclopentadiene derivative and arylphenoxide as fixed ligands around a transition metal, the arylphenoxide being substituted with at least one aryl derivative and being located at an ortho position thereof, and the ligands not being crosslinked to each other: wherein M is a 4 th group transition metal of a periodic table; Cp is a cyclopentadienyl anion capable of forming an η 5 -bond along with the central metal, or a derivative thereof; R 1, R 2, R 3, R 4, R 5, R 6, R 7, and R 8 of the arylphenoxide ligand are independently a hydrogen atom, a halogen atom, a C1-C20 linear or nonlinear alkyl group arbitrarily substituted with one or more halogen atoms, a silyl group which contains the C1-C20 linear or nonlinear alkyl group arbitrarily substituted with one or more halogen atoms, a C6-C30 aryl group arbitrarily substituted with one or more halogen atoms, a C7-C30 arylalkyl group arbitrarily substituted with one or more halogen atoms, a C1-C20 alkylalkoxy group arbitrarily substituted with one or more halogen atoms, or a C3-C20 alkyl-substituted siloxy group or C6-C20 aryl-substituted siloxy group, optionally with the proviso that substituent groups may be arbitrarily bonded to form rings; X is selected from or is two or more independently selected from a group consisting of the halogen atom, the C1-C20 alkyl group which is not the Cp derivative, the C7-C30 arylalkyl group, an alkoxy group which contains the C1-C20 alkyl group, the C3-C20 alkyl-substituted siloxy group, and an amido group which has a C1-C20 hydrocarbon group; Y is the hydrogen atom, the halogen atom, the C1-C20 linear or nonlinear alkyl group arbitrarily substituted with one or more halogen atoms, the silyl group which contains the C1-C20 linear or nonlinear alkyl group arbitrarily substituted with one or more halogen atoms, the C6-C30 aryl group arbitrarily substituted with one or more halogen atoms, the C7-C30 arylalkyl group arbitrarily substituted with one or more halogen atoms, the C1-C20 alkylalkoxy group arbitrarily substituted with one or more halogen atoms, the C3-C20 alkyl-substituted siloxy group or C6-C20 aryl-substituted siloxy group, the amido group or a phosphido group which has the C1-C20 hydrocarbon group, or a C1-C20 alkyl-substituted mercapto or nitro group; and n is 1 or 2 depending on an oxidation value of the transition metal, wherein the boron compound is expressed by Formula 8 or 9, and wherein the organic aluminum compound is expressed by Formula 10: Formula 8 [R 11] + [B(R 10) 4] - Formula 9 [(R 12) q ZH] + [B(R 10) 4] - wherein B is a boron atom; R 10 is an unsubstituted phenyl group, or a phenyl group which is substituted with 3 to 5 substituent groups selected from the group consisting of a C1-C4 alkyl group which is substituted or unsubstituted with a fluorine atom and a C1-C4 alkoxy group which is substituted or unsubstituted with the fluorine atom; R 11 is a C5-C7 cyclic aromatic cation or an alkyl-substituted aromatic cation; Z is a nitrogen atom or a phosphorus atom; R 12 is a C1-C4 alkyl radical or an anilinium radical which is substituted with two C1-C4 alkyl groups along with a nitrogen atom; and q is an integer of 2 or 3, Formula 10 (R 13) r Al 3-r wherein R 13 is a C1-C8 alkyl group, E is a hydrogen atom or a halogen atom, and r is an integer ranging from 1 to 3.