NOVELTY - A glass substrate comprises (in %mass) silica (53-72), alumina (1-15), magnesium oxide (0.5-9), calcium oxide (0.1-11), strontium oxide (0-11), barium oxide (0-11), sodium oxide (2-11), potassium oxide (4-21) and zirconia (0.5-10.5), in the section of 5000 nm or more in depth from the surface of glass substrate. The ratio of average sodium amount (atomic%) in section of 10-40 nm in depth from the surface of glass substrate before heat treatment at 600 degrees C for 1 hour under nitrogen atmosphere to the average sodium amount after heat treatment is 1.5 or more.    USE - Glass substrate is used for solar cell (claimed).    ADVANTAGE - The solar cell using glass substrate has excellent electric power generation efficiency and high glass transition temperature, and is economical.    DETAILED DESCRIPTION - A glass substrate comprises (in %mass) silica (53-72), alumina (1-15), magnesium oxide (0.5-9), calcium oxide (0.1-11), strontium oxide (0-11), barium oxide (0-11), sodium oxide (2-11), potassium oxide (4-21) and zirconia (0.5-10.5), in the section of 5000 nm or more in depth from the surface of glass substrate. The total content of magnesium oxide, calcium oxide, strontium oxide and barium oxide in the glass substrate is 4-25 %mass. The total content of calcium oxide, strontium oxide and barium oxide in the glass substrate is 4-23 %mass. The total content of sodium oxide and potassium oxide in the glass substrate is 8-22 %mass. The mass ratio of sodium oxide with respect to total content of calcium oxide, strontium oxide and barium oxide is 1.2 or less. The ratio of average total content (atomic%) of calcium, strontium and barium in the section of 10-40 nm in depth from the surface of the glass substrate to the total content of calcium, strontium and barium in the section of 5000 nm in depth from the surface of the glass substrate is 0.35 or less. The ratio of average sodium amount (atomic%) in section of 10-40 nm in depth from the surface of glass substrate before heat treatment at 600 degrees C for 1 hour under nitrogen atmosphere to the average sodium amount after heat treatment is 1.5 or more. The glass substrate has glass transition temperature of 580 degrees C or more, and average thermal expansion coefficient of 70x 10-7-100x 10-7/ degrees C. An INDEPENDENT CLAIM is included for solar cell (1).    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of solar cell.    Solar cell (1)    Glass substrate (5)    Positive electrode (7)    Transparent conductive film (13)    Negative electrode (15)