To provide a bath water modification device where space-saving is made possible when installing an electrolytic unit 10, and energy is effectively utilized to save electric power required for electrolysis.

This bath water modification device is provided with: the electrolytic unit 10 installed inside a bath 51 and through which the bath water to be modified circulates; a controller 20 applying voltage between electrodes 12 and 13 arranged inside the electrolytic unit 10 so as to be controlled; and a power generating apparatus 30 rotating a hydraulic turbine 30a by the flowing energy of hot water circulating through a water feed path 41 and generating power by a generator 30b. The controller 20 feeds electric power generated in the generating apparatus 30 to the electrolytic unit 10 and modifies the bath water into alkali water. Namely, the electrolytic unit 10 is installed inside the bath 51, so that space-saving can be realized. Further, heat generation is performed by utilizing the flowing energy at the time when water is fed to the bath 51 and the bath water in the bath is electrolyzed by the generated electric power, so that the effective utilization of the energy, i.e., energy-saving can be realized.