Abstract: A synoviolin expression inhibitor for inhibiting the expression of a synoviolin gene and a synoviolin protein is provided. A synoviolin expression inhibitor, which inhibits an expression of a synoviolin gene or an expression of a synoviolin protein, the inhibitor comprising, as an active ingredient, a mesenchymal stem cell or a culture supernatant thereof and a method of producing a synoviolin expression inhibitor comprising the step of obtaining a mesenchymal stem cell or a culture supernatant thereof by culturing a mesenchymal stem cell using a serum-free medium, and the step of preparing the synoviolin expression inhibitor, which inhibits an expression of a synoviolin gene or an expression of a synoviolin protein, by using the mesenchymal stem cell or the culture supernatant thereof obtained in the previous step.