Using the IRAM 30-m telescope, we have detected a rotating gas disk around the relatively old (5x106yr) T Tauri star DM Tau. The kinematic pattern and line profiles obtained from 12CO J=2->1 and 13CO J=2->1 are consistent with a disk inclined about 30deg from face-on, with a rotation axis at PA~=90deg, and orbiting a 0.65M☉ central star with a systemic velocity of 6.1km/s. All detected lines can be well fitted by a simple Keplerian disk model with outer radius ~=750AU, mean temperature about 15K, and standard isotopic ratios. If the CO abundance is normal, the total disk mass is 1.4x10–3M☉. Such a mass is small compared to that derived from mm and sub-mm continuum emission (~=0.03M☉). This can be explained either by CO depletion, or by an anomalous gas-to-dust ratio, or by the existence of a dense, compact, optically thick core which dominates the mm continuum emission. The size derived for such a core is consistent with the size of the current solar system. In all cases, our results indicate that significant amount of gas still exists several Myr after star formation.