An elastic fiber treatment agent that contains, as a smoothing agent, a mineral oil with a content ratio of an aromatic component of less than 1% by mass and an aniline point of 70° C. to 110° C. The mineral oil has a mass ratio between the content of a naphthene component and the content of a paraffin component of such that naphthene component/paraffin component=30 to 50/70 to 50.