Focusing methods using mirror optics are intensively studied in the field of X-ray microscopy because mirror optics has useful features such as high photon efficiency and no chromatic aberrations. Employing a wave-optical method, we investigated the relationship between the natures of figure errors on the mirror surface and optics performances such as sizes, intensities and satellite peak structures of the focused X-ray beam. We also evaluated uniformity of intensity in the spherically diverging beam from the focal point as a point source. Obtained results showed unprecedented degrees of surface figure accuracy such as higher than 1nm in all over the spatial wavelength range longer than 0.5mm was required to realize nearly diffraction-limited focusing including satellite structures and intensity flatness at the spherically diverging wavefront.