The human thumb is a digit playing crucial roles among all digits used for object manipulation. The special feature of the thumb termed as pad-opposition with four other fingers makes human precision manipulation possible, and during four other fingers dexterously manipulate an object, the thumb functions as a role of stably supporting other's manipulating motions within the pad-opposition. In various characteristics of the thumb, this paper concentrates upon the large radius of pad curvature of the thumb compared with those of four other fingers. In order to show the influence of the large pad curvature, it is shown that reiterated fingering motions can be easily generated by a pair of robotic fingers with different pad curvatures by using a sensory-feedback. It is also shown from computer simulations and experiments that the pad curvature automatically distinguishes roles of the thumb and other fingers. It is finally claimed that the thumb plays a crucial role in enhancement of dexterity and stability in such an implicit manner that it helps the other opposite finger with less pad curvature to make a swift rolling movement that can sophisticatedly control movements of the object.