We designed a hydrogel with a thermoresponsive crosslinked domain (CD) structure, which expressed thermoresponsive toughening in an isochoric manner in air. The responsive behavior of conventional thermoresponsive hydrogels is usually a macroscopic volume change along with water transfer to and from the outside of the polymer network. In order to expand the scope of thermoresponsive hydrogels, a network design that requires no external water for thermoresponsive transition has been demanded. To this end, we focused on the polymerization-induced self-assembly process for the direct synthesis of gels with designed domains and performed reversible addition-fragmentation chain transfer polymerization of N-isopropylacrylamide in water at a high temperature using a bifunctional macro-chain transfer agent, yielding a transparent gel. The structural analysis of the obtained gel revealed that dispersed CDs reversibly swelled and shrunk in response to temperature change in air without aggregation. Such an internal structural change induced the increase of elastic modulus and elongation at a high temperature.