PURPOSE: To reduce the central tensile stress and surface compression stress and prepare a glass plate without the self propagation of a breaking line even on the occurrence of cracks nor breaking into fine fragments, by heat-treating the glass plate under specific conditions.

CONSTITUTION: A glass plate 1, e.g. a window glass, of 5W15mm thickness is introduced into a roll hearth 2 and heated at 600W660°C by a heating apparatus 4 while conveyed by rolls 3 in the roll hearth 2. The glass plate 1 is then moved to a gap between the first cooling air blowing holes 5 for blowing cooling air, and the air is blown from the cooling air blowing holes 5 on the glass plate for 1W20sec to give the following value: hK≥45 [h(mm) is the thickness of the glass plate 1; K(°C/sec) is the cooling rate]. The cooled glass plate 1 is then transferred to a gap between hot plates 6 at 100W500°C temperature, annealed to 450°C or below and then finally cooled with air blown from the second cooling air blowing holes 7. Thus, the aimed glass plate having 85W200kg/cm2 central tensile stress (σt) and 1.5W3.0 ratio (σc/σt) between the surface compression stress (σc) and the central tensile stress (σt) is obtained.