A method for defining a fluid/solid boundary for computational fluid dynamics simulations making use of a coordinate system mesh and a solid object model, which includes the steps of defining straight lines on which all of the grid points of the coordinate system mesh are positioned; obtaining intersecting points of the straight lines with the surface of the solid object model; and a step in which, for each of the straight lines having the intersecting points, the grid points on the straight line are searched for the nearest point to each of the intersecting points, and based on the searched-out nearest points, the positioned grid points are determined as to whether the grid point is positioned inside the solid object model or in the fluid region.