Disclosed is a method for commercially advantageously producing a high-quality polyolefin with high yield in a uniform system in the presence of a catalyst which contains a transition metal compound as the main catalyst component. Specifically disclosed is method for producing a polyolefin by polymerizing one or more substances selected from ethylene and a-olefins having 3-30 carbon atoms in a saturated hydrocarbon solvent in the presence of a catalyst containing a catalyst component (A) composed of a compound of a group 4 transition metal of the periodic table, and a catalyst component (B) composed of an organic aluminoxane. This method for producing a polyolefin is characterized in that the component (B) is obtained by subjecting an organic aluminoxane, which is soluble in a saturated hydrocarbon solvent, to a contact treatment with an organic aluminum compound having an alkyl group with 1-10 carbon atoms in advance in the saturated hydrocarbon solvent.