Characterizing the chemistry of complex organic molecules (COMs) at the epoch of planet formation provides insights into the chemical evolution of the interstellar medium (ISM) and the origin of organic materials in our solar system. We report a detection of dimethyl ether (CH₃OCH₃) in the disk around the Herbig Ae star MWC 480 with sensitive Atacama Large Millimeter/submillimeter Array observations. This is the first detection of CH₃OCH₃ in a nontransitional Class II disk. The spatially unresolved, compact (≲25 au in radius) nature, broad line width (∼30 km s^‑1), and high excitation temperature (∼200 K) indicate the sublimation of COMs in the warm inner disk. Despite the detection of CH₃OCH₃, methanol (CH₃OH), the most abundant COM in the ISM, has not been detected, from which we constrain the column density ratio of CH₃OCH₃/CH₃OH ≳ 7. This high ratio may indicate the reprocessing of COMs during the disk phase, as well as the effect of the physical structure in the inner disk. We also find that this ratio is higher than in COM-rich transition disks recently discovered. This may indicate that in the full disk of MWC 480, COMs have experienced substantial chemical reprocessing in the innermost region, while the COM emission in the transition disks predominantly traces the inherited ice sublimating at the dust cavity edge located at larger radii (≳20 au).