To investigate the patients’ consciousness regarding the use of metformin and medical cost in Japanese patients with type 2 diabetes taking metformin. This cross-sectional study investigated patients’ general characteristics and consciousness regarding medical cost, kidney function, liver function, and metformin usage, using a self-administered, internet-based questionnaire. Among 1,000 patients, 81.0% felt unsatisfied with treatment for type 2 diabetes, with the main reason for dissatisfaction being high medical cost, with 540 patients reporting this. In addition, 16.8% of patients experienced treatment disruption and among them, 48.2% (81/168) answered that the reason was high medical cost. Over half of the patients did not understand their kidney and liver functions, respectively. Only 8.9% and 7.1% of patients knew both the words and meanings behind sick days and lactic acidosis, respectively. In conclusion, many patients with type 2 diabetes taking metformin were not satisfied with their treatments, with the main reason being high medical cost. Moreover, they did not have sufficient knowledge of sick days and/or lactic acidosis.