This study investigates elastic behavior attributed to flexor digitorum tendon, using ultrasonographic images. The tendon is stretched during finger extension both by muscle strength and external forces applied to the fingertip. The elastic force exerted by the stretched tendon can work as torque for flexsion of the finger. Our experimental scheme includes measuring changed MP joint angle and each corresponding exerted fingertip force and displacement of image feature on flexor digitorum in the ultrasonographic image. The measurement results of the relationship among the exerted fingertip force, flexion angle and ultrasonographic images show that extension or displacement of the tendon is proportional to the flexion angle. In addition, a hysteresis of the fingertip force in stretching and shortening cycle can be observed. The hysteresis addressed in the related studies may be caused mainly by behavior of flexor digitorum muscles.