An optical system for X-ray holographic microscopy has been developed. The optical system consists of a double-prism X-ray interferometer and an X-ray imaging microscope with a Fresnel zone plate objective. One prism is placed at the object plane of the microscope, and the other is set at the back focal plane of the objective lens. The first prism separates the reference wave from the object wave, and the second prism deflects the reference wave in order to superimpose it onto the magnified image of the object. Then, a spherical wavefront of the reference wave is formed at the image plane, and Fresnel diffraction at the boundary of the prism is well suppressed in the measured hologram, because an in-focus image of the first prism is formed on the image plane, and the reference wave and the object wave are separated from each other in the back focal plane. Experimental results at the beamline 20XU of SPring-8 are shown.