  We present our proposal for a small X-ray mission DIOS (Diffuse Intergalactic
Oxygen Surveyor) to perform survey observations of warm-hot intergalactic
medium using OVII and OVIII emission lines. This will be proposed to a small
satellite program planned by ISAS/JAXA in Japan for a launch in 2008. The
instrument consists of an array of TES microcalorimeters with an energy
resolution 2 eV, cooled by mechanical coolers. The X-ray telescope will employ
4-stage reflection mirrors with a focal length as short as 70 cm and an angular
resolution 2'. In addition to DIOS, we briefly describe the NeXT (New X-ray
Telescope) mission, which is a larger Japanese X-ray observatory to be launched
in 2010 and plans to explore non-thermal processes in the universe.
