NOVELTY - A glass substrate (5) contains 50-72 %mass silica, 1-15 %mass alumina, 0-10 %mass magnesium oxide, 0.1-11 %mass calcium oxide, 0-13 %mass strontium oxide, 0-11 %mass barium oxide, 1-11 %mass sodium oxide, 2-21 %mass potassium oxide and 0-10.5 %mass zirconium oxide, and has sum total content of magnesium oxide, calcium oxide, strontium oxide and barium oxide of 4-25 %mass, at depth of 5000 nm or more from the surface. The glass substrate has glass transition temperature of 580 degrees C or more and average thermal expansion coefficient of 70x 10-7/ degrees C to 100x 10-7/ degrees C.    USE - Glass substrate is used for copper-indium-gallium-selenium solar cell (claimed).    ADVANTAGE - The glass substrate has high glass transition temperature. The copper-indium-gallium-selenium solar cell comprising the glass substrate has excellent power generation efficiency.    DETAILED DESCRIPTION - A glass substrate has amount of calcium, strontium and barium in surface layer of glass substrate in inner portion of 0.7 atomic% or less at depth of 5000 nm or more from the surface, ratio of sodium oxide content (%mass) measured by fluorescence X-rays from glass substrate surface and sodium oxide content (%mass) measured by fluorescence X-rays from surface at depth of 5000 nm from surface of 0.4-1.1, and ratio of sodium in glass-substrate surface layer before heat treatment and after heat treatment at 600 degrees C for 1 hour under nitrogen atmosphere of 1.1 or more at depth of 10-40 nm (from the glass-substrate surface). At depth of 5000 nm or more from glass-substrate surface, the glass substrate contains 50-72 %mass silica, 1-15 %mass alumina, 0-10 %mass magnesium oxide, 0.1-11 %mass calcium oxide, 0-13 %mass strontium oxide, 0-11 %mass barium oxide, 1-11 %mass sodium oxide, 2-21 %mass potassium oxide, 0-10.5 %mass zirconium oxide and has sum total content of magnesium oxide, calcium oxide, strontium oxide and barium oxide of 4-25 %mass, sum total content of calcium oxide, strontium oxide and barium oxide of 2-23 %mass and ratio of content of sodium oxide and sum total content of calcium oxide, strontium oxide and barium oxide of 1.2 or less. The glass substrate has glass transition temperature of 580 degrees C or more and average thermal expansion coefficient of 70x 10-7/ degrees C to 100x 10-7/ degrees C. An INDEPENDENT CLAIM is included for solar cell (1).    DESCRIPTION OF DRAWING(S) - The drawing shows the sectional view of the solar cell.    Solar cell (1)    Glass substrate (5)    Buffer layer (11)    Transparent conductive film (13)    Anti-reflective film (17)