Extra Ultraviolet Scanner (XUV) measure the abundance and distribution of helium gas and helium ions in the Martian ionosphere by scanning the scattered extreme ultraviolet sunlight by neutral he lium gas and helium ions. The neutral helium gas observation gives us the information on the internal activity of Mars :e.g. volcanic activity and/or water circulation. On the other hand, helium ions measurement will give us the information on the ionization proces s of helium gas and its escape from the ionosphere to outer space.