PROBLEM TO BE SOLVED: To provide a novel and useful substance that promotes formation of tight junction related to adhesion or sealing between cells, or a substance which induces expression of tight junction protein 1, or a substance which induces cell membrane localization of the tight junction protein 1.SOLUTION: A tight junction formation promoter, a tight junction protein 1 expression-inducing agent, a tight junction protein 1 cell membrane localization inducer, and furthermore, other applications (medicine, drink and food, skin external preparation) have fruit skin of mandarin orange or extract of fruit as an active ingredient.SELECTED DRAWING: None