A refined product of a polyoxypropylene fatty acid isopropanolamide surfactant, having a high stability in storage and a very small change in smell thereof, is obtained by heat-treating an addition-reaction mixture prepared from a compound of the formula (III) and propylene oxide in a molar amount of 0.5 to 10 times that of the compound of the formula (III), and containing the target compound of the formula (I) and an oxazoline compound of the formula (II) produced as a by-product with water or an alkaline aqueous solution at 50 to 100.degree. C., to hydrolyze the oxyazoline compound and to control the content of the oxyazoline compound to 0.1% by mass or less; and a detergent composition containing the refined product in a content of 1 to 50% by mass, exhibits enhanced thickening, foaming, emulsifying, dispersing and dissolving properties. ##STR00001## [R.sup.1.dbd.C.sub.1 C.sub.19alkyl or alkenyl group, X=oxypropylene group, n=average number of addition-reacted X groups of 0.5 to 10]