Fast radio bursts (FRBs) are millisecond transients of unknown origin(s) occurring at cosmological distances.Since the first discovery of FRB, more than 100 FRBs have been detected. Numerous theoretical models of FRB progenitors have been proposed to date. However, the origin(s) is still unknown. One way to constrain the FRB progenitors is to search for their burst counterparts in different wavelengths at the positions of FRBs. Another way is to investigate stellar populations of galaxies hosting FRBs. In this work, we use ALMA data dedicated to FRB121102, which is the most famous 'repeating' FRB hosted by a dwarf star-forming galaxy at z=0.19. No clear multi-wavelength burst counterpart of this FRB has been reported to date. We report the first ALMA detection of FRB121102 at 90.5 and 104.5 GHz during its "active phase". The detected source is either the burst counterpart or host galaxy.These possibilities will be further discussed in our presentation.