This paper discusses an underactuated robotic hand for grasping a free-flying object. We introduce a concept of creating a closure called caging before locking the target in an immobilizing grasp. The caging is a closure that geometrically constrains the target such that the target can not escape from its closure created by robotic fingers. The design of the underactuated hand that performs caging-based grasping of the target is presented. The proposed underactuated hand was evaluated through the static analysis and under a condition called object closure. The prototype of the proposed hand was developed, and its performance was verified experimentally using an air-floating system.