ALMA has revealed fine structures such as ring disks and crescent asymmetries in tens of protoplanetary disks. Planets are believed to form in such dense regions, and thus, these disks with fine structures are prime targets to understand planet formation. To date, most fine structures have been detected at r >5 au (e.g., HL Tau) where gas giant planets could form. On the other hand, ring structures at r <5 au are relatively rare (e.g., TW Hya). Since rocky planets could form in the inner region of the snowline, observations of such small ring disks at r <5 au are important to understand planet formation at terrestrial planet forming regions. In this talk, I report the case study of DM Tau (a mass of 0.5 Mo) observed with ALMA cycle 5 & 6 at 1.3mm. We found the inner dust ring at r =3.2 au, which potentially corresponds to the main asteroid belt in our Solar system. We also found that the north part of the inner disk is about 20% brighter than that of the south part in cycle 5 data. Intriguingly, in cycle 6 data, we found a possible bright blob that rotates about 270 degrees. This blob is about a 20 sigma detection. The time interval between cycle 5 and 6 observations is about 1.6 yr. Thus, this blob is unlikely to be a vortex because local Keplerian rotation at r =3 au cannot explain a 270 degree rotation in 1.6 yr. On the other hand, this blob might be a structure induced by an unseen planet at r =1.3 au.