In this study, we propose and develop a system which early detects dementia in elderly people living alone. In the proposed system, sensors obtain behavior of elderly people living alone. Then, suspicion of dementia is determined by calculating a score of sensed behavior. Two different analytical methods are applied to determine suspicion of senile dementia. In the experiment, Suitable analytical methods were determined for this system in environment by pseudo patients with dementia. And, adequacy of analytical method was evaluated by the experimental result. Moreover, we evaluated the system by the questionnaire to test subjects.