It is difficult to confirm a diagnosis of early‒onset Alzheimer’s disease（EOAD）because patients sometimes have non‒specific cortical features, such as psychiatric symptoms, executive functional impairment, and pyramidal symptoms, along with typical symptoms, such as recent memory impairment and disorientation. We encountered a patient with multiple psychotic symptoms, finally diagnosed with EOAD on genetic testing. A right‒handed sixty‒year‒old man, whose mother was suspected of having dementia, developed memory impairment at the age of fifty, disorientation at the age of fifty-six, and both visual hallucination and dressing apraxia at the age of fifty-nine. After admission to a psychiatric hospital for treatment, his symptoms disappeared with antipsychotic medication. However, his ADL were declining and so he was referred to our university hospital. He had frontal lobe symptoms, pyramidal signs, and extrapyramidal signs with severe dementia. Neuropsychological examinations were not possible because of sedation. On brain MRI, he showed diffuse atrophy of the cerebral cortex and hippocampus. HMPO‒SPECT showed hypoperfusion of cerebral cortices diffusely. We decided to perform genetic testing because he had both family and alcohol abuse histories. He showed EOAD with V717I mutation of the amyloid precursor protein gene. After the discontinuation of antipsychotics, excessive sedation and extrapyramidal signs disappeared. A dose of 10 mg of donepezil was effective to improve motivation and activity, and his mini mental examination score was calculable after recovery. The case supports usefulness of applying genetic testing for Alzheimer’s disease to patients with early onset dementia, even when they do not have a family history.