This paper describes effects of viscous liquid 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 of kinetic viscosity 1000 mm^2/s were provided to the pads, and normal adhesive forces between the pad and a glass surface were measured. The experimental results show that the pad provided with the silicone oil generates higher adhesive force than the pad with water. The adhesive force of the silicone oil remains constant regardless of the pad materials and amount of the oil, and increases with increasing pull-off velocity.