The present invention provides a closed-cell foamed rubber sheet whose sealability is less likely to decrease even after long-time use and which has excellent water resistance. The closed-cell foamed rubber sheet of the present invention comprises a rubber-based resin, and is provided by the steps of: providing a foamable resin composition containing the rubber-based resin and a foaming agent; irradiating the foamable resin composition with ionizing radiation to cross-link the foamable resin composition; and foaming the cross-linked foamable resin composition. In the case where the foamable resin composition further contains 0.5 parts by weight or more of a powdery additive other than the foaming agent per 100 parts by weight of the rubber-based resin, the rubber-based resin preferably contains a rubber-based resin ingredient which is in a liquid state at normal temperature.