The present invention provides a heat-foamable sheet that foams uniformly and omnidirectionally, a method for producing the heat-foamable sheet, and a foam filling member including the heat-foamable sheet. A heat-foamable sheet having a horizontal to vertical ratio of 1.5 or less when heated at 160° C. for 20 minutes is obtained by a method for producing a heat-foamable sheet including: an extrusion step of extruding a heat-foamable material containing a polymer and a foaming agent into an isotropy-containing shape including an isotropic portion of a generally arc shape; and a sheet-forming step of forming the heat-foamable material that is extruded in the extrusion step into a sheet shape.