PROBLEM TO BE SOLVED: To provide a novel tight junction formation promoter.SOLUTION: The feature of the invention is as follows. (1) A tight junction formation promoter contains a swallow's nest extract as an active ingredient. (2) A claudin-1 protein expression promoter contains a swallow's nest extract as an active ingredient. (3) A claudin-1 gene-expression promoter contains a swallow's nest extract as an active ingredient. (4) A claudin-4 protein expression promoter contains a swallow's nest extract as an active ingredient. (5) A claudin-4 gene-expression promoter contains a swallow's nest extract as an active ingredient.