In a case of repairing an aortic valve by forming a valve cusp with a biomembrane, the shape and the size of a valve cusp can be determined easily. An instrument including a valve cusp sizer for measuring the size of the valve cusp and a template for scribing lines for the shape of a valve cusp conforming to the measured size of the valve cusp, in which the valve cusp sizer has a plurality of differently-sized sizer blocks attached to each of the top ends of grip members respectively and each sizer block has an arcuate surface formed by cutting each of differently-sized circular cylinders at an angle conforming to the central angle for the commissure portions and, the template is formed with a scribing portion including a substantially semi-circular valve cusp base forming portion.
