While motor imagery has been known as a powerful tool for neuro-rehabilitation in stroke patients, whether this technique is also effective for other brain disorders is unclear. For instance, patients with Parkinson's disease or attention-deficit hyperactivity disorder who are impaired at real motor switching may benefit therapeutically from training that consists of switching their imagined motor movements, and eventually recover from the dysfunction. However, despite its importance little is known about exactly how switching mental images of one's actions is processed in the brain. Therefore, we set out to clarify this issue by measuring brain activity reflected in electroencephalograms as subjects switched an imagined hand rotation from one hand to the other during a motor-imagery task. By comparing electroencephalogram signals from repeated mental imaging of hand movements, we found a switch-specific decrease in the beta-band activity in parietal and frontal regions around 0.6 s after stimulus presentation. Further, we found rotation-related negativity in the parietal cortex at the same time as the decreased beta-band power. These results suggest that the parietal area is dynamically involved in the switching of imagined hand motion, and that frontal areas may have an important role in inhibiting mental imagery of the deselected hand's motion.