PROBLEM TO BE SOLVED: To provide a denaturation/degradation inhibitor capable of inhibiting denaturation and degradation of protein due to a radical (for example, resulting from a radical generated by light irradiation such as ultraviolet irradiation and exposure to active oxygen), and a denaturation/degradation inhibitor-containing composition including the denaturation/degradation inhibitor.SOLUTION: A denaturation/degradation inhibitor for inhibiting denaturation and degradation of protein due to a radical includes glucopyranosyl glycerol (A1) and/or a derivative (A2) thereof as an active ingredient.