To provide a gap-filling foamable sheet which does not exhibit the decrease in viscosity in heating and hence does not allow the flow down of the resin of the sheet when stuck to a vertical plane and which is excellent in shape retention before and after thermal foaming.

This gap-filling foamable sheet, which foams and cures under heating, is formed from an epoxy resin compsn. contg. an epoxy resin liq. at normal temp., a curative for an epoxy resin, a blowing agent, and a cross-linked phenoxy resin obtd. by reacting a phenoxy resin with a polyisocyanate, subject to the conditions that the amt. of the phenoxy resin for preparing the cross- linked phenoxy resin is 5-20wt.% of the sum of the phenoxy resin and the epoxy resin and that the ratio of isocyanate groups of the polyisocyanate to hydroxyl groups of the phenoxy resin is 0.05-1.0.