We thought to investigate the influence of DPP-4 inhibitor on the serum BNP level of diabetic patients with congestive heart failure. This study consists of two protocols. In the first protocol, diabetic patients with high level of serum BNP (100-2000 pg/ml) receiving outpatient treatment with any type of DPP-4 inhibitor at our institution will be enrolled. After baseline data collection, DPP-4 inhibitor will be suspended for one month and serum BNP will be measured. The DPP-4 inhibitor will be resumed and after another month, serum BNP will be measured again. In the other protocol, diabetic patients who were hospitalized due to heart failure were enrolled. DPP-4 inhibitor will be started after stabilization of heart failure, and serum BNP will be measured before and after the drug administration.