The aim of this paper is to propose a novel robotic hand exoskeleton which can assist disabled people with grasping objects. Existing exoskeletons cannot measure force during contact motion, and this makes it difficult to achieve accurate grasp assistance. In the proposed system the exoskeletal finger is attached to the index and middle fingers, and the robotic finger is place on the palm side, which functions as a substitute of a thumb. The upper exoskeletal finger exerts assistive force on operator's fingers, and the lower robotic finger works as a force sensor by the use of Reaction Torque Observer. Thus, human grasping force is measured during grasping, and the environmental and human forces are estimated from measured torque. The proposed system realizes power assist control without using additive sensors like force sensors or EMG sensors. The effectiveness of the proposed method is demonstrated through simulation and experiment.