Personal robots and Robot Technology (RT)-based assistive devices are expected to play a major role in Japan's elderly-dominated society, both for joint activities with their human partners and for participation in community life. These new devices should be capable of smooth and natural adaptation and interaction with their human partners and the environment, should be able to communicate naturally with humans, and should never have a negative effect on their human partners, neither physical nor emotional. To achieve this smooth and natural integration between humans and robots, we need first to investigate and clarify how these interactions are carried out. Therefore, we developed the portable Bioinstrumentation System WB-2 (Waseda Bioinstrumentation system No.2), which can measure the movements of the head, the arms, and the hands (position, velocity, and acceleration), as well as several physiological parameters (electrical activity of the heart respiration, perspiration, pulse wave, and so on), to objectively measure and understand the physical and physiological effects of the interaction between robots and humans. In this paper we present our development of the Inertial Measurement Unit, which is at the heart of our new motion-capture system that replaces the system used in the Waseda Bioinstrumentation system No.1 Refined (WB-1R). Some preliminary results of experiments with the unit are also presented and analyzed.