In situ observations using interference contrast microscopy have carried out in an electrochemical deposition system of a ZnSO4 solution film. Concentration oscillation around a growing dendrite tip, which is illustrated by local contrast oscillation, has been found to correspond to sidebranching of the dendrite. A model concerning the competition between diffusion of cations and growth of dendrites has been used to explain this oscillation phenomenon. It appears that this concentration field oscillation in front of the dendrite tip plays an important role in sidebranching.