The Report deals with a characteristic flow field around a slender rectangular thin wing, where free vortices, which separate from side edges, roll up and form a symmetrical spiral vortex sheet. The aims are to obtain lift distribution and to grasp the macroscopic flow field around the wing. Referring to the observed flow pattern around the wing by means of a hydrogen bubble method in a water tunnel, a simple flow model is assumed. The model consists of a pair of core vortices and feed vortex sheets which emerge from side edges to the core vortices. With the simplification of the flow field, boundary conditions need to be made less detailed. That is, the condition on the wing surface is satisfied only along the center line of it, and the condition for the free vortices is simplified to the condition that total forces acting on the free vortices are zero. By the method, reasonable results are obtained for normal force coefficient, center of pressure and average location of core vortices.