The both-terminal-functional polyolefin, useful in various applications, is represented by general formula (I) (wherein X and Y are each a group containing at least one kind of element selected from among oxygen, sulfur, nitrogen, phosphor, and halogens, provided X and Y may be the same or different; P is a polymer chain formed by using, as the main component, an olefin composed of only carbon atoms and hydrogen atoms; and X is bonded to one terminal of P, and Y, to the other terminal) and its molecular weight distribution (Mw/Mn) obtained by gel permeation chromatography (GPC) is 1.0-1.5.