This paper reports an ongoing project of preventive care for elder citizens with dementia by using a smartphone app. Identification of early signs of the disease is critical for prevention and treatment. We focus on an early sign of dementia that the elderly often show in daily life when they operate electronic devices. We describe a design of a smartphone app that detects the specific sign and screens the elderly in the very early stage of dementia. The app is a privacy-friendly tool of dementia prevention for families or relatives of the elderly who live remote from each other. Our app is expected to provide mHealth researches with big data collection platform.