A variable valve system varies an operation condition of an engine valve by controlling an angular position of a control shaft in accordance with an operation condition of the engine. The system has an actuating mechanism for actuating the control shaft. The actuating mechanism comprises a threaded shaft that is rotated about its axis in accordance with the operation condition of the engine; a nut member operatively engaged with the threaded shaft, so that upon rotation of the threaded shaft the nut member runs axially along the threaded shaft; a link mechanism provided between the control shaft and the nut member, so that the axial movement of the nut member along the threaded shaft induces a rotational motion of the control shaft; and a biasing mechanism that biases the nut member relative to the threaded shaft at least at a predetermined range of the operation condition of the engine valve.