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DIOS (Diffuse Intergalactic Oxygen Surveyor) is a small satellite aiming for a launch around 2022 with JAXA's Epsilon rocket. Its main aim is a search for warm-hot intergalactic medium with high-resolution X-ray spectroscopy of redshifted emission lines from OVII and OVIII ions. The superior energy resolution of TES microcalorimeters combined with a wide field of view (30 diameter) will enable us to look into gas dynamics of cosmic plasmas in a wide range of spatial scales from Earths magnetosphere to unvirialized regions of clusters of galaxies. Mechanical and thermal design of the spacecraft and development of the TES calorimeter system are described. Employing an enlarged X-ray telescope with a focal length of 1.2 m and fast repointing capability, DIOS can observe absorption features from X-ray afterglows of distant gamma-ray bursts.