The purpose of the present invention is to provide a hydrogel, which has excellent adhesiveness with electrode elements, having an optimal adhesive force to skin surface and having corrosive resistance to corrosive objects of aluminum, tin and the likes which is used as the electrode elements.A hydrogel 1 having an A layer 10 and a B layer 20 is characterized by that the A layer contains a component originated from a monomer, water, a moisturizer and an electrolyte, the chloride ion concentration in the A layer 10 is 2.0 weight % or lower with respect to the water, the B layer 20 contains a component originated from a monomer, water, a moisturizer and an electrolyte, and pH of the hydrogel is 3 to 7.本發明之目的在於提供一種與電極元件之接著性優異，對於皮膚表面具有最適的黏著力，且同時對於使用作為電極元件之鋁或錫等腐蝕對象物具有腐蝕耐性之水凝膠。一種具有A層(10)與B層(20)之積層構造之水凝膠(1)，其特徵為：A層(10)中含有：來自單體之成分、水、保濕劑及電解質，A層(10)中的氯化物離子濃度相對於前述水為2.0重量%以下，B層(20)中含有：來自單體之成分、水、保濕劑及電解質，且水凝膠之pH為3~7。
