  We present Atacama Large Millimeter/submillimeter Array (ALMA) observations
of the dust continuum emission at 1.3 mm and 12CO J=2-1 line emission of the
transitional disk around DM Tau. DM Tau's disk is thought to possess a
dust-free inner cavity inside a few au, from the absence of near-infrared
excess on its spectral energy distribution (SED). Previous submillimeter
observations were, however, unable to detect the cavity; instead, a dust ring
~20 au in radius was seen. The excellent angular resolution achieved in the new
ALMA observations, 43 x 31 mas, allows discovery of a 4 au radius inner dust
ring, confirming previous SED modeling results. This inner ring is symmetric in
continuum emission, but asymmetric in 12CO emission. The known (outer) dust
ring at ~20 au is recovered and shows azimuthal asymmetry with a strong-weak
side contrast of ~1.3. The gap between these two rings is depleted by a factor
of ~40 in dust emission relative to the outer ring. An extended outer dust disk
is revealed, separated from the outer ring by another gap. The location of the
inner ring is comparable to that of the main asteroid belt in the solar system.
As a disk with a "proto-asteroid belt," the DM Tau system offers valuable clues
to disk evolution and planet formation in the terrestrial planet forming
region.
