It is intended to provide a method for measuring cholesterol in remnant-like lipoprotein in a sample with a simpler procedure and with high sensitivity, and an assay reagent. The method for measuring cholesterol in remnant-like lipoprotein, in which cholesterol in lipoprotein is measured by allowing a cholesterol esterase and a cholesterol oxidase or a cholesterol dehydrogenase to act on a test sample containing lipoprotein and measuring hydrogen peroxide or a reduced coenzyme generated by the enzymatic reaction, characterized by using a surfactant having a benzenesulfonic acid structure in its molecule or a polyacrylic acid polymer surfactant.