Caging is a method to make an object inescapable from a closed region by rigid bodies. Position-controlled robot hands can capture an object and manipulate it via caging without force sensing or force control. However, the object in caging is freely movable in the closed region, which may not be allowed in some applications. In such cases, grasping is required. In this paper, we propose a new simple approach to grasping by position-controlled robot hands with the advantage of caging: caging-based grasping by a robot hand with rigid and soft parts. In caging-based grasping, we cage an object with the rigid parts of the hand, and construct a complete grasp with the soft parts. We formulate the caging-based grasping, and derive concrete conditions for caging-based grasping in planar and spatial cases, and show some experimental results.