PURPOSE: To prevent the formation of dangerous splines when sheet glass splinters by heating a flat glass of a small thickness then air-cooling the specific range thereof from the top and rear surface to form areas having higher compressive stress and lower compressive stress separately.

CONSTITUTION: A thin flat glass 9 of 2.4W4.0mm thickness is heated to 610W700°C and is conveyed by means of guiding members 10 to between many upper spraying nozzles 5 and lower spraying nozzles 8. Thereafter cooling air is sprayed from upper and lower spray nozzles onto the top and rear surface of the plate 9 to form areas 11 where surface compressive stresses are as large as 1,300kg/ cm2 and areas 12 where said stresses are as small as 1,020kg/cm2 at 10W30mm intervals in a belt shape. The differences in the compressive stresses between both surfaces are set at 80W220kg/cm2 and the max. main stress difference in the areas 12 where the surface compressive stresses are small is set at ≥80kg/ cm2. When broken, this flat glass splinters to 5×5cm sizes and this flat glass is utilized for window glass of automobiles as tempered glass that does not produce any dangerous and sharp splines.