To surely measure the velocity of the blood flow within a papilla part with good accuracy.

First the portions where light is most intensive and the portions where the light is weakest are determined on all areas of an area sensor 59 and whether all the areas on the area sensor 59 attain the nearly uniform intensity of the light or not is judged by determining the differences therebetween. If the light can be judged to be uniform, the absence of the bold blood vessel on the area sensor 59 can be judged. When the light cannot be judged to be uniform, the portion where the light is weak is determined and whether the portion of the weak light is the bold blood vessel or not is judged by determining the ratio thereof on the area sensor 59. When the portion of the weak light is below 10% of the entire part, the influence on the scattered light from the papilla part by this area is considered to be substantially nil and, therefore, the scattered light from the inside of the papilla part En is subjected to a frequency analysis and the velocity of the blood flow is determined by considering that the measurement in the position absent of the bold blood vessel is executed.