The study aimed to establish new criteria for diagnosing 'obesity disease' in Japan, focusing on obesity-related complications. The research involved 1,193 Japanese subjects (775 men, 418 women) with a BMI range of 14.9 to 56.4 kg/m². Visceral fat area (VFA) and subcutaneous fat area (SFA) were measured using CT scans, and anthropometric parameters such as BMI, waist circumference (W), waist-to-hip ratio (W/H), and waist-to-height ratio (W/BH) were assessed. Hyperglycemia, dyslipidemia, and hypertension were evaluated as obesity-related complications. The results showed that the number of complications increased with BMI, with an average value exceeding 1.0 at a BMI of 25. The best combination of sensitivity and specificity for detecting multiple risk factors was at a BMI of 25. BMI showed a strong positive correlation with SFA but a weaker correlation with VFA. The number of disorders was greater than 1.0 at a VFA of 100 cm², and the best combination of sensitivity and specificity for determining multiple risk factors was at this VFA. The waist circumference (W) had the closest relationship with VFA, with a regression line indicating that the W corresponding to 100 cm² of VFA was 84.4 cm in men and 92.5 cm in women. The study concluded that obesity should be defined as BMI ≥25 in Japan, and VFA ≥100 cm² is a reliable indicator of obesity-related disorders. The waist circumference of 85 cm in men and 90 cm in women approximates to this visceral fat mass. These findings provide a basis for new guidelines for diagnosing 'obesity disease' in Japan.The study aimed to establish new criteria for diagnosing 'obesity disease' in Japan, focusing on obesity-related complications. The research involved 1,193 Japanese subjects (775 men, 418 women) with a BMI range of 14.9 to 56.4 kg/m². Visceral fat area (VFA) and subcutaneous fat area (SFA) were measured using CT scans, and anthropometric parameters such as BMI, waist circumference (W), waist-to-hip ratio (W/H), and waist-to-height ratio (W/BH) were assessed. Hyperglycemia, dyslipidemia, and hypertension were evaluated as obesity-related complications. The results showed that the number of complications increased with BMI, with an average value exceeding 1.0 at a BMI of 25. The best combination of sensitivity and specificity for detecting multiple risk factors was at a BMI of 25. BMI showed a strong positive correlation with SFA but a weaker correlation with VFA. The number of disorders was greater than 1.0 at a VFA of 100 cm², and the best combination of sensitivity and specificity for determining multiple risk factors was at this VFA. The waist circumference (W) had the closest relationship with VFA, with a regression line indicating that the W corresponding to 100 cm² of VFA was 84.4 cm in men and 92.5 cm in women. The study concluded that obesity should be defined as BMI ≥25 in Japan, and VFA ≥100 cm² is a reliable indicator of obesity-related disorders. The waist circumference of 85 cm in men and 90 cm in women approximates to this visceral fat mass. These findings provide a basis for new guidelines for diagnosing 'obesity disease' in Japan.