Taking the Si(111)square-root 3 X square-root 3-Ag surface as an example, it has been investigated how the atomic arrangement of the probe tip affects obtained scanning tunneling microscope (STM) images using first-principles theoretical calculations. Various shapes of the tip were represented by tilting the symmetry axes of four kinds of clusters consisting of several tungsten atoms. Tips having a few equivalent atoms at their apexes produce distinctly different STM images from experiment if their symmetry axes are normal to the sample surface. Even in these cases, however, calculated STM images become similar to the observed ones by tilting the symmetry axes. This fact indicates that only one atom at the tip apex gives essential contribution to the tunneling current. Furthermore, it can be explained why good images can be obtained, in spite of the fact that the shape of experimental STM tips must be quite arbitrary on the atomic scale.