To provide a glass fiber-composited resin substrate, which has a high-level heat resistance and transparency, and has a sufficiently low thermal expansion coefficient.

The glass fiber-composited resin substrate is composed of a curable resin composition and a glass fiber. The curable resin composition contains (A) a cage-type silsesquioxane resin having at least one group selected from the group consisting of a (meth)acryloyl group, a glycidyl group and a vinyl group, (B) an unsaturated compound having ≥2 unsaturated functional groups selected from the group consisting of groups represented by formula (1) and (2):-R¹-CR²=CH₂...(1) and -CR²=CH₂...(2), and (C) a curing catalyst, wherein the content of (A) the cage-type silsesquioxane resin is 5 to 90 mass% with respect to the whole of the curing resin composition.