A process for producing a crosslinked and foamed resin sheet which comprises shaping a resinous mixture comprising a thermoplastic synthetic resin and a heat-decomposable blowing agent into a sheet form, applying a high-energy ionizing radiation to the resulting foamable resin sheet to crosslink the resin sheet, and then heating the resulting crosslinked foamable resin sheet to the decomposition temperature of the blowing agent or to a higher temperature; wherein the high-energy ionizing radiation has a range of at least 0.7 .rho.d g/cm.sup.2 in which d is the thickness in cm of the foamable resin sheet, and .rho. is the density in g/cm.sup.3 of the foamable resin sheet, and a low-energy ionizing radiation with a range of not more than 0.6 .rho.d g/cm.sup.2 in which d and .rho. are as defined is applied to the foamable resin sheet before, during or after the application of the high-energy ionizing radiation.