Numerical simulation is performed for steady state flows in a driven cavity at low Reynolds numbers. The flows of incompressible viscous fluid are driven by the top lid of a cubic cavity which slides at constant speed in directions not parallel to the sidewalls. The Reynolds number Re and the angle of lid motion theta are adopted as the governing parameters. Structure of streamlines is studied by means of the Poincare map and the rotation number N of the tori formed by streamlines. The rotation number N of the tori is given as a function of (Re, theta). The appropriateness of using N as an index of mixing efficiency is discussed.