In food science, natural ingredients that can inhibit dipeptidyl peptidase IV ( DPP IV ) may be useful for preventing diabetes mellitus. This study aimed to evaluate the effectiveness of bonito dashi having a high DPP IV inhibitory activity (IC50 ; 3049 µg/ ml) on the postprandial increase in blood glucose levels in 14 subjects. Bonito dashi (5 g) was subsequently subjected to oral glucose tolerance tests. Blood glucose levels of all subjects were measured at fasting and at 30 min after ingesting of bonito dashi or of warmed tap water as a control, and were also measured at 30, 45, 60, 75, 90, 120, and 150 min after ingestion of cooked rice. The maximum blood glucose level between 0 and 150 min after ingesting of bonito dashi was for 30 min, of warmed tap water for 45 min ). The blood glucose levels after ingestion of bonito dashi and warmed tap water were 135.6 ± 8.7 mg/ dl and 140.3 mg/ dl, respectively at 30 min ( p = 0.602 ); 135.6 ± 8.7 mg/ dl and 144.1 ± 10.7 mg/ dl, respectively at 45 min ( p = 0.057 ); 120.0 ± 4.9 mg/ dl and 136.8 ± 7.8 mg/ dl, respectively at 60 min ( p = 0.063 ); 110.0 ± 5.9 mg/ dl and 134.9 ± 6.9 mg/ dl, respectively at 75 min ( p = 0.006 ); 110.3 ± 6.8 mg/ dl and 129.3 ± 6.6 mg/ dl, respectively, at 90 min ( p = 0.036 ); 103.4 ± 4.1 mg/ dl and 118.7 ± 8.0 mg/ dl, respectively, at 120 min ( p = 0.091 ); 91.5 ± 3.8 mg/ dl and 102.3 ± 5.9 mg/ dl, respectively, at 150 min ( p = 0.232 ). The area under the curve for blood glucose levels after ingestion of bonito dashi and warmed tap water was 4753.1 ± 439.7 mg/ dl ×min and 6879.4 ± 728.1 mg/ dl ×min, respectively ( p = 0.005 ). Postprandial increase in blood glucose levels was lower in subjects ingestion of bonito dashi than in those ingestion of the warmed tap water. No serious adverse events related to ingestion of bonito dashi were observed. Our findings suggested that the ingestion of bonito dashi (5 g) suppressed postprandial increase in blood glucose levels in our subjects.