Soft materials, including elastic polymers and hydrogels, are promising materials for soft robots, implantable devices, and wearable devices owing to their flexibility and biocompatibility. In recent years, the scope of use of such soft materials is expanding further because of their integration with metal micro- and nanostructures, which exhibit attractive optical and electrical properties. In this study, we demonstrate the formation of metal microstructures inside a hydrogel by photoreduction of metal ion by using femtosecond laser. Because the hydrogels have permeability to fluids, metal ions are able to be added by simply immersing the hydrogel into the metal ion solution. Femtosecond laser pulses were focused and scanned in the hydrogel to fabricate metal microstructures [1, 2]. The optical diffraction pattern obtained with the silver grating inside the hydrogel showed spaced diffraction spots, which indicated that a regular, periodic silver grating was formed. Notably, the distance between the diffraction spots changed when the water content in the hydrogel was reduced. The grating period decreased when the hydrogel shrank owing to the loss of water (Fig. 1(a)), but the straight shapes of the line structures were preserved, which demonstrated the optical tunability of the fabricated structure.