Double-network (DN) hydrogels have attracted considerable attention owing to their unique mechanism to show extraordinary mechanical strength and toughness. Although the toughening mechanism of the DN gels, breaking of the relatively stiff and brittle first network as sacrificial bonds, is widely accepted, the microstructure and morphology evolution of the internal damage have hardly been revealed. In this study, we study the internal structures of the first network in partially damaged DN gels by using the microelectrode technique (MET) based on the Donnan effect of the polyelectrolyte first network. We measure the spatial distribution of the electric potential of the prestretched and then reswelled DN gels. From the anisotropic depth profiles of potential and reswelling ratio, the microstructures of DN gels at a scale larger than the microelectrode probe size (similar to 200 nm) are revealed at the preyielding, yielding, and strain-hardening regimes.