A new type of common path interferometer using a modified zone-plate developed for testing aspheric surfaces is described. The modified zone-plate is made by combining two kinds of zone-plates; the semicricular part of the plate produces the spherical wave, and the remainder the desired aspheric wave. These are synthesized by digital computation. A method of computation for synthesizing the modified zone-plate and some experimental results of the test of parabolic mirrors are given.