A newly found phenomenon, in which the electric resistivity in a highly purified water gradually decreases through hours and days after application of a milli-gauss ac magnetic field in a short time, its mechanism model, and its wide application for various positive biological effects are summarized. A theoretical model for the mechanism based on the cyclotron resonance with the frequency f=qB_dc/2πm for each hydro-molecular cluster with the proton charge q and the cluster mass m of H₃O⁺(H₂O)_n under the terrestrial field B_dc well explains the gradual decreasing ratio of the resistivity for each cyclotron frequency forming a long range order proton transport path. The high temperature stability of a magnetized water resistivity may reflect the homeostasis. The phagocytic activity of neutrophils is remarkably elevated in a magnetized phosphate-buffered saline (PBS) solution. The milli-gauss, ULF magnetic field applied water will be a basic medium for the magneto-protonics.