The old gray cast iron for water supply pipes in Yokohama City which was made in the UK in 1887 is highly corrosion resistant. The corrosion behavior of the old gray cast iron in soils was studied. For comparison, a gray cast iron used for modern pipes, a spheroidal graphite cast iron, and a carbon steel were tested. The corrosion potentials and corrosion rates were measured. The corrosion products and the base metals were analyzed by X-ray diffractometry and energy dispersion spectroscopy. The corrosion rate of the gray cast iron made in the UK in 1887 was lower than those of the others. The gray cast iron contains a large amount of steadite, which is a ternary eutectic (γ-Fe, Fe3C, and Fe3P). The network structure of steadite enclosed the corrosion products and decreased the corrosion rate.