Aiming to expand the range of applications for parallel grippers, we propose an extension nail mechanism that can be mounted on a parallel gripper. We also propose an extension nail mechanism comprising a stainless steel belt, two transport belts, a triangular nail, and a drive unit. The triangular nail is connected to one end of the stainless steel belt, and the drive unit is connected near the other end. We achieve smooth sliding of the nails underneath objects by arranging the transport belts on either side of the stainless steel belt. By elastically winding one end of the stainless steel belt and each of the transport belts, the nail mechanism can be miniaturized while achieving large expansion and contraction. We achieve stable grasping operations by using the extension nail mechanism of the parallel gripper in accordance with the flexibility of the object.