A sealing material including a water-resistant sheet, wherein the water-resistant sheet includes layered clay minerals having a thickness of 0.5 nm to 800 nm. A sealing material including a sheet, wherein the sheet includes modified layered clay minerals in which at least a portion of a first cation between the interlayer of swellable layered clay minerals is ion-exchanged with a second cation, in a first cation being one or more selected from Na+ and Li+. A sealing material including a sheet, wherein the sheet includes layered clay minerals having a thickness of 0.5 nm to 800 nm, and having one or more selected from K+, Ba2+ and Pb2+ are contained in at least a portion in an interlayer of the clay minerals.