Pressure-induced decubitus is a serious disease among the elderly people. Interface pressure occluding vascular perfusion is known to be a cause of decubitus. Therefore, it is essential to quantify the relationship between vascular perfusion and interface pressure among the elderly people to understand more about decubitus. Nine healthy elderly people (57.8$pm$ 5.6 years, 63.3$pm$ 7.0kg, 1.68$pm$ 0.05m) were participated. Three healthy young people (31.7$pm$ 3.2 years, 74.7$pm$ 8.4kg, 1.75$pm$ 0.04m) were also examined to be compared with the elderly group. Capillary vascular perfusion on the ischial tuberosity was recorded in the sitting posture as pressures were applied from 15mmHg to 135mmHg. The average interface pressure to occlude vascular perfusion (the average occlusion pressure) under the ischial tuberosity was 115.7mmHg in the elderly group. This value was not significantly different from the average occlusion pressure of the young group. Obesity effect on the occlusion pressure was investigated among the elderly group. The result was not significantly different between the obesity and the normal group in this study. This is a preliminary study to unveil the complicated cause of pressure-induced decubitus associated with occlusion of vascular perfusion. More subjects are required for the future study.