Musician's dystonia (MD) is a task-specific movement disorder characterized by abnormal posturing and loss of fine motor control. Typically, the affected fingers become uncontrollable only during playing a specific musical instrument, and respond normally to other motor activities. Pathophysiology of MD still remains unclear although some studies have claimed excessive structural and functional reorganization of the brain. To gain insights into the neural basis of MD, we conducted behavioral and fMRI studies. In a behavioral experiment, we asked 14 experienced healthy pianists and 15 pianists with MD to execute simple tapping sequences on an electronic piano at 4 different tempi. MIDI data was acquired. All pianists with MD had dystonic symptom in right hand or both hands. Then, we asked the same groups of participants to execute the same tapping sequence at 200 ms inter-keystroke interval (IKI) during functional MRI acquisition. We tested both hands in the behavioral experiment, and right hand only in the fMRI experiment. In the behavioral experiment we found that variance of both IKI and key-depressing velocity was larger in both hands in pianists with MD than healthy pianists. Mean key-depressing velocity was smaller in pianists with MD than in healthy pianists. Note that the key-depressing velocity is approximately proportional to strength of keystroke, determining sound volume. The results from the behavioral experiment suggested that pianists with MD were not able to stroke keys strongly or keep constant tempo. When brain activity was compared across the groups, we found that activation in the right cerebellum was significantly greater in pianists with MD than in healthy pianists. In addition, we asked pianists with MD whether dystonic symptom appeared or not during tapping sequence task in the MRI scanner. The cerebellar activation in pianists with MD who had dystonic symptom was greater than those in pianists with MD who did not. Thus, in the MD group, the cerebellar activity reflected presence of dystonic symptoms during functional MRI. The results suggest that exaggerated cerebellar activity might be related to pathophysiology of MD.