A multifingered hand can reorient an object in a hand. This paper discusses one of primitive ways to reorient a prism object. Two fingers (corresponding to the human thumb and middle fingers) grasp the top and bottom faces of the prism object. Their fingertips form a rotational axis. The third finger (corresponding to the human index finger) exerts a force to the side faces to rotate the object about the axis. Since the workspace of the third finger is limited, the side faces reachable by it are also limited depending on the position of the axis formed by the first two fingers. This paper shows an algorithm for computing the possible region for the axis so that the third finger can rotate the object from start to goal angles. The algorithm also determines the side faces to be pushed by the third finger. It is applicable to any shaped prism objects.