An integration-based temporal-optimization is proposed for the determination of optical flow. The method is based on the continuity equation of fluids. Dynamic scenes are regarded as a kind of hydrodynamic flow (called image flow tentatively). A generalized expression of the continuity equation for the image flow is shown explicitly. The continuity image sequences under assumptions of incompressible fluids without sources or sinks and of temporal constancy of the flow field. Improvements in accuracy and reliability of the method is confirmed by simulation experiments.