The design of multi-branch optical waveguides having 3 or more output ports is not so easy as that of 2-output branches because some innovative geometry is required to realize equal power splitting. All previous studies take the same approach in which they first introduce innovative geometries and then adjust the structural parameters for equal splitting. On the other hand, we propose quite a different method where distribution of refractive index is calculated from an ideal field distribution which is synthesized artificially. The method is extended to design 3-D 4-branch waveguides. It is exemplified that 4-branch waveguides with low-loss and equal splitting can be realized by the proposed method.