Muscle contraction occurs by mutual sliding between thick (myosin) and thin (actin) filaments. But the physical and chemical properties of the sliding force are not clear; even the precise direction of sliding force generated at each cross-bridge is not known. We report here the use of a recently developed in vitro motile assay system to show supercoiling of an actin filament in which the front part of the filament was fixed to a glass surface through cross-linked heavy-meromyosin and the rear part was able to slide on a track of heavy-meromyosin. A left-handed single turn of superhelix formed just before supercoiling, suggesting that the sliding force has a right-handed torque component that induces the right-handed rotation of an actin filament around its long axis. The presence of the torque component in the sliding force will explain several properties of the contractile system of muscle.