Diet treatment for diabetes requires restriction of the food amount (energy intake). It is desirable that patients have a proper relative consumption of the three main nutrients (proteins, carbohydrates, fats) and also habitually take low-energy foods such as vegetables, mushrooms and seaweeds, etc. as often as possible in each meal. Therefore, we can replace the expression ‘a diet for diabetes’ with ‘a diet for healthy living’. By showing a clinical case of an obese diabetic patient, who succeeded to reduce their body weight, HbA1c and oral agents through diettreatment, and finally could go on diet treatment only, the importance of diet therapy can be emphasized. Furthermore, the estimation index was examined to evaluate how accurately diabetic patients could estimate food energy. According to this study, a large amount of food on the plate leads most patients to underestimate the amount of real energy, and patients are apt to eat too much compared with having smaller amounts of food on the plate. By analyzing questionnaires on the diet therapy of approximately 3000 diabetics, it has been shown that the majority of patients at our hospital recognize that diet therapy is the most important factor in the treatment of diabetes. Interestingly, patients who ate all the food served showed a significantly higher body mass index compared with those who left served food.