Adjustability for grasping force, which is one of motor function of fingers, is an ability to grasp an object with appropriate force. A person with a disable hand cannot grasp an object with appropriate force and always grasps an object by using extra force. This paper proposed a quantitative evaluation system of the adjustability for grasping force. The system consists of a grasping device, an amplifier box and a computer. The grasping device is called iWakka. The system displays the grasping force and the desired one on the monitor. The desired grasping force is an up and down step-wise pattern. A subject grasps iWakka and lifts it up. While watching the grasping force and the desired one on the monitor, a subject adjusts the grasping force so as to make it agree with the desired one as possible. The system computes the evaluation value of the adjustability for grasping force. The effectiveness of the proposed system was verified for ten healthy persons.