There had been few reports of arachnoid cyst accompanying psychiatric disturbance and no report treated with low-dose risperidone administration. We report a patient, developed first-transient psychotic episode considered to be provoked by an arachnoid cyst, responsive to risperidone, which was found in the middle cranial fossa as follows. A 57-year-old man was admitted by suddenly developed headache, auditory hallucination, delusion of persecution and, an arachnoid cyst in the anteromedial aspect of middle cranial fossa was found on MRI after admission. The psychotic episode was first to him and he was also negative to other clinical evaluation including endocrine abnormality, his psychotic symtom was suspected to be induced by arachnoid cyst and was well controlled to low-dose risperidone administration. He left hospital free from psychotic symptoms on 14 hospital days.