An adhesive hydrogel has low contact resistance against the body surface and excellent conformability and adhesiveness to the body surface, so that noise generation can be reduced. Such adhesive hydrogel enables measurements of weak fetal heart rate signals. The adhesive hydrogel includes a polymer matrix prepared by crosslinking-copolymerization of an acrylamide derivative, which is a nonionic polymerizable monomer including a polymerizable carbon-carbon double bond in a molecule, and polyfunctional (meth)acrylamide, which is a crosslinkable monomer comprising 2 or more polymerizable carbon-carbon double bonds in a molecule, water, a polyvalent alcohol, and an electrolyte salt, wherein a laminate of the adhesive hydrogel with an Ag/AgCl sheet exhibits the impedance of 1Ω to 100Ω at the frequency of 0.1 Hz measured in accordance with the method of ANSI/AAMI EC12:2000, and the dynamic elastic modulus G′ at the frequency of 1 Hz measured at 25° C. and dispersed frequencies is 1.0×103 Pa to 1.0×104 Pa.