Metabolic syndrome becomes a serious problem in our life, because it is one of the reasons of arteriosclerosis, heart disease or stroke. In order to prevent a person from becoming a metabolic syndrome, it is important to know own current actual abdominal circumference, and care by yourself. However, any wearable devices cannot measure abdominal circumference. In this paper, we propose and implement a belt-type wearable device, called "WaistonBelt", and a smartphone application that can keep monitoring an abdominal circumference forever. "WaistonBelt" can measure an abdominal circumference with ordinary action, and can visualize a transition of it. Through an experimental evaluation, we confirm that our system can measure an abdominal circumference with 0.66% error.