The author improved the accuracy of the direct colori metric method of calcium determination with azocal-A using an electrophotometer and made an investigation of interfering ions concerning this method. With this improved method calcium content of 0.2-several mgs per liter can be determined using only 5cc. of river water sample with an error of 土10%. Only 2 minutes are enough for a single determination and this method surpassed the ordinary oxalate method in accuracy.