Abstract: Raw water G containing arsenic in an amount exceeding an environmental standard value is poured into a treatment bath filled with particulate carriers. The raw water G in the treatment bath is treated at a flow rate which does not allow production of ferric hydroxide in a suspended form in the raw water while adding an acidic iron solution to the raw water G so that the pH value of the raw water G is adjusted to 6.5-7.5. A ferric hydroxide membrane is produced on the entire surfaces of the carriers by contact oxidation reaction of dissolvable ferrous ions in the raw water G having the adjusted pH value, to cause the arsenic in the raw water G to be adsorbed on the ferric hydroxide.