This paper describes effects of viscous force on adhesive characteristics of sole pads for insect-inspired wall-climbing robots.Ants can walk on the smooth vertical surfaces using an adhesive organ and secreted viscous liquid.We fabricated two kinds of adhesive pads inspired by the adhesive organ of ants.Pure water and silicone oil with various kinetic viscosities were provided to the pads,and normal and tangential adhesive forces between the pad and a glass surface were measured.The experimental results show that the pad provided with highly viscous silicone oil generates higher adhesive force than the pad utilizing surface tensionforce of water.