A hydrogel of the semi-interpenetrating network structure or interpenetrating network structure, characterized in that the hydrogel is one formed from a first monomer ingredient at least 10 mol % of which is accounted for by a charge-possessing unsaturated monomer, the molar ratio of the first monomer ingredient to the second monomer ingredient being from 1/2 to 1/100, and that the second monomer ingredient has been polymerized and crosslinked to a lower degree of crosslinking than the first monomer ingredient. The hydrogel is usable in a diaper, sanitary good, sustained-release agent, construction material, building material, communication material, soil conditioner, contact lens, ocular lens, hollow fiber, artificial cartilage, artificial organ, material for fuel cells, battery diaphragm, impact-resistant material, cushion, etc.