1. A method for numerical analysis of a flow field of incompressible viscous fluid, directly using V-CAD data, comprising: a dividing step (A) of dividing external data into a plurality of cells ( 13 ) having boundaries orthogonal to each other, the external data including boundary data of an object which contacts incompressible viscous fluid; a cell classifying step (B) of classifying the divided cells into an internal cell ( 13 a ) positioned inside or outside the object and a boundary cell ( 13 b ) including the boundary data; a cut point determining step (C) of determining cut points in ridges of the boundary cell on the basis of the boundary data; a boundary face determining step (D) of determining a polygon connecting the cut points to be cell internal data for the boundary face; and a analyzing step (E) of applying a cut cell finite volume method combined with a VOF method to a boundary of a flow field to analyze the flow field.