A hybrid method is under development to measure 3-dimensional velocity fields where the image processing of visualized flow and numerical computations are used. The 3-dimensional velocity is computed from several scanned plane flows which have been obtained by the flow visualization and image processing. It is necessary to analyze the plane flow fields as accurately as possible, for they are used as boundary conditions in computations for the 3-dimensional reproduction. The present paper proposes a new method to determine the plane flow velocity accurately. The method is to analyze the intensity distribution profiles of visualized streaklines rather than the discrete images of tracers. The velocity is determined from the movement of the profiles at three successive times. The zero crossing points are invoked to characterize the profiles whose identification is much easier than the tracer method because of its 1-dimensional identification along the streakline. The use of the distribution of the intensity rather than the binary data makes the results accurate where the results do not depend on the slice level. Because the intensity distribution profile carries much information than discrete binary data, amendment for higher accuracy can be done by making use of the correlation function and so on. The method is applied to analyze the uniform flow and the plane flow around the stern of Wigley model. Although several points are still left to be improved for higher accuracy, the present method provides much better plane flows than those obtained by the particle-trace method. It is expected to be used for the computation of 3-dimensional flow reproduction.