A boundary calculating unit is provided for determining boundaries between separated plasma and blood cell or air and plasma whose image is picked up by an image pickup unit. With such boundary calculating unit provided, the boundaries between the separated plasma and blood cell or air and plasma can be determined, and areas of the separated air, plasma, and blood cell can be determined accurately. Particularly, a straight line drawing unit is provided for drawing, on the image picked up, a plurality of straight lines parallel to grooves in a disk which performs plasma separation. The boundary calculating unit can easily determine the boundaries by determining the boundaries based on a profile of the plurality of straight lines drawn by the straight line drawing unit.
