A method of manufacturing a resin foam includes dissolving a blowing agent into a mixture of a resin C and a resin D having a difference of solubility with respect to the blowing agent in an amount of more than 0% and less than 5%; subjecting the mixture to heat-melting for extrusion; and evaporating the blowing agent such that the resin foam has a porosity (X) of not less than 80%, a cell (L) with a cell diameter of not less than 1 μm and not more than 1000 μm, and a cell (S) with a cell diameter of not less than 0.01 μm and less than 1 μm, in which two peaks are present in cell diameter distribution, one peak thereof being present in not less than 10 μm and not more than 500 μm and the other peak being present in not less than 0.01 μm and less than 1 μm.