An area S (m2) of a facing surface of each of injectors which faces a glass ribbon is set so as to satisfy: S≦(0.0116×P×Cg×T)/{ε×F×σ(Tgla4−Tinj4)}, wherein P is an output (ton/day) of the glass ribbon; Cg is a specific heat (J/(kg·° C.)) of the glass; T is an acceptable temperature drop (° C.); ε is radiation factor; F is a surface-to-surface view factor; σ is Boltzmann's constant; Tgla is a temperature (K) of the glass ribbon represented by K=(Tin+Tout)/2 where Tin and Tout are measured values of the glass ribbon at the inlet and outlet of the injector, respectively; and Tinj is a temperature (K) of the facing surface of the injector.