A focal atonic seizure is a partial seizure in which the ictal manifestation consists of paresis of the extremities or muscles on one side of the body, and this phenomenon can easily be misdiagnosed as a transient ischemic attack. An 86-year-old woman visited our hospital complaining of transient right upper extremity weakness lasting for 10 min following an unusual sensation in her chest accompanied by palpitations. On the third hospital day, she again complained of right arm weakness, which progressed to jerky movements of her right extremity accompanied by facial twitching and then generalized into a tonic–clonic seizure. The EEG displayed several interictal spikes in the contralateral temporal area, and the ictal SPECT, analyzed using the SISCOM system, showed an increased signal in both the contralateral superior parietal area and the mesial frontal area. In this case, the patient was diagnosed with focal atonic seizures as the cause of the monolimb weakness, which had been initially misdiagnosed aas transient ischemic attacks. In cases in which a patient presents with monolimb paresis, physicians should consider the possibility of an atonic seizure as the cause.