NOVELTY - Cover glass (I) comprises a compressive-stress layer on its principal surface, where the cover glass has a glass composition containing 50-70 mole% of silicon oxide, 3-20 mole% of aluminum oxide, 5-25 mole% of sodium oxide, 0-2.5 mole% of lithium oxide, 0-5.5 mole% of potassium oxide and 0-3 mole% of boron oxide.    USE - The cover glass is useful as strengthened glass (claimed) for protecting display screens of portable electronic equipment e.g. mobile phones and digital cameras, components for protecting solar cells, and components in casings of mobile terminals.    ADVANTAGE - The cover glass provides compressive-stress layer that can be measured easily and produced with improved ion exchangeability in short processing time and mechanical strength and devitrification resistance.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) producing the cover glass (I), comprising: preparing molten glass by melting a glass raw material; forming the prepared molten glass into a plate-like shape by a down-draw process, thus obtaining a glass substrate having the glass composition; and forming a compressive-stress layer on the surface of the glass substrate;    (2) a strengthened glass comprising the compressive-stress layer on its principal surfaces, where the strengthened glass has the glass composition;    (3) a cover glass (II) comprising the compressive-stress layer on its principal surfaces, where the cover glass has the glass composition containing 45-70 mass% of silicon oxide, 5-25 mass% of aluminum oxide, 9-25 mass% of sodium oxide, 0-2.5 mass% of lithium oxide, 0-3 mass% of boron oxide and 0-10 mass% of magnesium oxide; and    (4) producing the cover glass (II), comprising: preparing molten glass by melting the glass raw material; forming the prepared molten glass into the plate-like shape by the down-draw process, thus obtaining the glass substrate with the glass composition; and forming the compressive-stress layer on the surface of the glass substrate.