Although cerebellar infarction can be confirmed by head computed tomography (CT), it could be easily misdiagnosed in the emergency department. In recent years, a head CT scan is often performed with thin slices, except for cases where stroke is suspected, which requires thin and thick slices to avoid misdiagnosis. A 54-year-old man with poorly controlled diabetes mellitus visited another emergency department 1 day after experiencing sudden nausea, vomiting, and unsteadiness. Upon examination, including head CT, the cause of the symptoms remained unclear. Although nausea was alleviated, unsteadiness remained and, therefore, he was admitted to his family clinic because of unsteady walking 4 days before visiting our hospital. Although he was still receiving treatment in the clinic, the unsteady walking remained and, therefore, he presented to our hospital. On presentation, the patient demonstrated ataxia while in a standing position. On head CT, left cerebellar peduncle infarction was detected and confirmed by images reconstructed from thick slices. Therefore, physicians should use both thin and thick slices of head CT images for patients suspected of cerebral infarction. In addition, CT scans should be reexamined when abnormal neurological findings, such as truncal ataxia, are observed.