High blood glucose level (BGL) is related to the high viscosity of blood. Red blood cells (RBCs) tend to gather each other in the condition of high viscosity. Backscattered echoes become high as RBCs aggregate. Therefore, the intensity of backscattered echo is anticipated to be high as the BGL increases. In the present study, we investigated the relationship between RBC aggregations by ultrasound and the BGL for the development of non-invasive BGL measurement methods. Ultrasonic backscattering echoes for dorsal hand vein and BGLs were repeatedly measured from fasting to 190 min after injecting 40-g glucose for a healthy subject. The brightness of the B-mode image increased as the BGL increased. The brightnesses of B-mode image and the BGLs after 160-190 min were less than those at fasting. The relationship between the brightness of the B-mode image and BGL was almost linear, although the hysteresis characteristic was observed. This is because multiple factors other than BGL are closely related to the RBC aggregations.