The purpose of this study is to assess the effect of Skinoren® 15% gel compared to no therapy on the maintenance of clinical therapy success in subjects with mild to moderate acne vulgaris previously treated for 3 months by a monotherapy of this substance and to assess the effect of Skinoren® 15% gel, compared to Differin® 0.1% gel on the efficacy and safety during a 9-month long-term treatment period.