Abstract: The present invention provides a rubber-based resin closed-cell foam sheet with its sealing capability being unlikely to be lowered even for a prolonged period of use, and with its excellent waterproofing capability. The inventive rubber-based resin closed-cell foam sheet is characterized in that the foam sheet includes a rubber-based resin containing a nitrile group, and exhibits an excellent adhesion. When used as a waterproof sealing material, the foam sheet can adhere, in a tightly close contact, to the interface of a portion to be sealed. Therefore, the inventive rubber-based resin closed-cell foam sheet can be advantageously used as a waterproof sealing material having high sealing capability and excellent waterproofing capability.