Approximately 30 to 50 percent of the total baryons in the present universe is supposed to take a form of warm/hot intergalactic medium ( WHIM ) whose X-ray continuum emission is very weak .
In order to carry out a direct and homogeneous survey of elusive cosmic missing baryons , we propose a dedicated soft-X-ray mission , DIOS ( Diffuse Intergalactic Oxygen Surveyor ) .
The unprecedented energy resolution ( \sim 2 eV ) of the XSA ( X-ray Spectrometer Array ) on-board DIOS 
enables us to identify WHIM with gas temperature T = 10 ^ { 6 - 7 } K and overdensity \delta = 10 - 100 located at z < 0.3 through emission lines of O vii and O viii . DIOS , hopefully launched in several years , promises to open a new window of detection and characterization of cosmic missing baryons , and to provide yet another important and complementary tool to trace the large-scale structure of the universe .