In this paper, we review the definition and the criteria of obesity and obesity disease in Japan. Obesity is defined for the subjects who are more than 25kg/m2 of body mass index (BMI). When such subjects have any diseases associated with obesity, they are diagnosed as obesity disease by the criteria decided in Japan Society for the Study of Obesity. According to the guideline for treatment and management of obesity disease 2006, obesity diseases are classified in two types of the following; one type is based on the qualitative changes of adipocytes (obesity disease with predominantly visceral fat or, so called, a type of metabolic syndrome), and the other is done on the quantative changes (obesity disease with predominantly subcutaneous fat). Obesity diseases with predominantly visceral fat are associated with diabetes mellitus, dyslipidemia, hypertension, and so far. On the other hand, obesity diseases with predominantly subcutaneous fat tend to induce osteoarthritis, obstructive sleep apnea syndrome (OSAS), and so far.