The author has made the study in order to calculate the calcium and magnesium intake from the daily drinking water of the citizens. Calcium and magnesium contents of 22 samples of supply water and 396 samples of well water in Tokyo-to and other area were determined. The results were as follows: 1. The calcium content of well water in wards (in Japanese “Ku”) was 1-117 p.p.m.(the mode was 16 p.p.m. for 24 samples) and magnesium content was 1-70 p.p.m.(the mode was 7 p.p.m. for 45 samples). 2. The calcium content of well water in countys was 6-30 p.p.m.(the mode was 14 p.p.m. for 35 samples) and magnesium content was 1-11 p.p.m.(the mode was 4 p.p.m. for 33 samples). 3. The calcium content of supply water was 14 p.p.m. and 20 p.p.m. From these results the calcium intake of a man from water was estimated to be about 30 mg daily.