The present invention provides a hydrogel which has excellent adhesion to an electrode element, and even if it absorbs moisture such as perspiration or moisture, the adhesion and adhesiveness to the electrode element or the object are only slightly reduced, and at the same time, has an optimum adhesive strength against the skin surface, and does not damage the skin when peeled off or the like.The hydrogel (1) of the present invention has a laminated structure of an A layer (10) and a B layer (20), wherein: the A layer (10) contains a monomer-derived component, water, a humectant, a water-insoluble polymer having adhesiveness, and an amphiphilic solvent-based polymer, said water-insoluble polymer contains 3 to 20 % by weight relative to the total amount of A layer, said amphiphilic solvent-based polymer is a polyvinyl alcohol having a degree of saponification of 50 to 75 % and contains 0.05 to 5 % by weight relative to the total amount of the A layer； the B layer (20) contains a monomer-derived component, water and a humectant, and substantially does not contain a water-insoluble polymer having adhesiveness and polyvinyl alcohol, the amount of said water relative to the total amount of B layer is ± 10% by weight of the amount of water relative to the total amount of A layer, and the amount of said humectant relative to the total amount of B layer is ± 10% by weight of the amount of humectant relative to the total amount of A layer.本發明之目的在於提供一種與電極元件之接著性優異，且即使吸收汗或濕氣等之水分，與電極元件或對象物之黏著性、接著性的降低程度小，同時對於皮膚表面具有最適的黏著力，於剝離時等不會對皮膚造成損傷之水凝膠。本發明為一種具有A層(10)與B層(20)之積層構造之水凝膠(1)，在A層(10)中，含有來自單體之成分、水、保濕劑、具有黏著性之水不溶性高分子以及兩親溶劑性高分子，前述水不溶性高分子相對於A層全量含有3至20重量%，前述兩親溶劑性高分子為皂化度50至75%的聚乙烯醇，且相對於A層全量含有0.05至5重量%；在B層(20)中，含有來自單體之成分、水以及保濕劑，且實質上不含具有黏著性之水不溶性高分子以及聚乙烯醇，相對於B層全量之前述水的量係相對於A層全量之水的量之±10重量%，相對於B層全量之前述保濕劑的量係相對於A層全量之保濕劑的量之±10重量%。
