Abstract: There is provided a retainer for maintaining an alignment of a first tooth and a second tooth adjacent to the first tooth. The retainer is configured in a form of a metal wire. The retainer includes: a first contact portion coming into contact with at least a part of a surface of the first tooth; a second contact portion coming into contact with at least a part of a surface of the second tooth; and a spaced portion. The spaced portion extends from the first and second contact portions, is located between the first and second contact portions, and is spaced from the first and second teeth. The spaced portion is located between an embrasure configured between the first and second teeth and an interdental space existing between the first and second teeth, and at least one edge of the spaced portion is a parabola having a small curvature.