This paper describes the image processing of visualized streak lines. Two-dimensional steady jet flow from the restriction of a spool valve is visualized by the hydrogen bubble method. The photographs of streak lines are taken with a 35 mm camera, and the coordinates of streak lines are fed into a microcomputer using a digitizer. Since streak lines coincide with streamlines for steady flow, a streamline coordinate system is used to conveniently calculate free streamlines of the free jet flow. A flow net, consisting of streamlines and curves orthogonal to the streamlines, is constructed. Finite difference approximation is applied to the basic equations of the streamline coordinate system. The velocity, vorticity and pressure at each node of the flow net are calculated using finite difference equations. To estimate the accuracy, these values are substituted into the equations of continuity and metric coefficient. The calculated results of residuals show that the present method is useful to analyze such a flow with free streamlines.