Generally, an autonomous under vehicle (AUV) is equipped with some navigation devices to know its own position and motion. One of them is an inertial navigation system (INS). The INS calculates its own absolute position and attitude using just internal sensors in real time. And the position accuracy depends strongly on precision of inertial sensors. In particular, the precision of optical gyroscopes strongly influences the position and attitude accuracy. And moreover, the precision depends on the size of them. In fact, it is necessary that the size of INS becomes large in order to improve the position accuracy. Based on that, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) developed a compact and high-performance INS which can be installed easily into a small AUV and output accurate position and attitude. It was developed with homegrown technology which is caused by JAMSTEC's knowledge and the technical capabilities of a domestic manufactures. In this paper, its schema, specifications and functions are described.