This paper introduces contribution of the thumb sensation to precisions of human's pinching force control when the human cooperatively pinch an object with the exoskeleton, which augments human pinching force based on a surface electromyography. A human hand should contact with a grasping object directly, while the exoskeleton pushes the human hand for grasping support, because tactile feedback from the hand is very important for human to handle an object stably and dexterously. However the exoskeleton may crush the human fingers with excessive force. The exoskeleton distributes the assistive force to the pinching object in two routes: directly and through a human finger. In our previous study, the accuracy of the pinching force is improved when the distribution factor of the index finger assistance keeps constant. This paper reports improvement of the pinching force when the fixed ratio of the pinching force assistance is applied to the index finger and thumb. In addition, we reveal the contribution of the thumb sensation.