This diffractometer was manufactured for research of solid surfaces and thin
 film surfaces and interfaces. To observe the X-rays diffracted from the
 surface of a sample in any direction, this instrument has a four-axis
 structure comprising 2+2 axes. With an ultra-high vacuum tank that allows
 on-the-spot observation, the axes are assembled so that the displacement of
 the rotating centers is less than 50??m. The UHV diffractometer has two degrees of freedom (DOF) on the sample and two independent DOF on the detector. The diffractometer is about 3.2 m x 3.2 m x 2.3 m in dimension. This instrument is now in service in the SPring-8 (Super Photon ring-8 GeV) BL13XU in Japan.