Accretion disks arc rotating disks formed by accreting matter onto compact objects, and are thought to be the source of various active phenomena. Dwarf novae, the best-studied disk systems, composed of a white dwarf with a surrounding disk and a red-dwarf companion star, exhibit double-peaked emission-line profiles because of disk rotation. When a disk is eclipsed by the companion star, the emission-line profiles vary with time, since a different portion of the disk having a different line-of-sight velocity is successively obscured. We report on spectacular variations of the line profiles during an eclipse that we obtained for the first time through Subaru spectroscopy of an eclipsing dwarf nova, IP Peg. Our fine data are useful for investigating detailed velocity profiles as well as the non-axisymmetric emissivity distribution. The observed variations are basically consistent with Keplerian rotation, but are more complex than those predicted by a simple axisymmetric model.