In this study, we developed a system for measuring the shape of a wearer’s abdominal circumference using a belt-type wearable device for the daily assessment of metabolic syndrome. The force applied to the belt from the abdomen was estimated using force sensors, and the shape of the abdominal circumference when the belt was not worn was estimated by considering the effect of outward force on the belt shape estimation. Using the developed device, we conducted evaluation experiments on eight adult males: two with a thin body type (BMI < 18.5), four with a standard body type (18.5 ≤ BMI < 25), and two with an overweight body type (25 ≤ BMI). The results showed that it was possible to estimate the shape of the nonwearing abdominal circumference with an average error of 3.73%. As a result of comparing the sensor values in the reference situation and the state in which each state changed, it was clarified that the system could detect the upright and exhale states appropriate for measurement.