The present invention provides a boiler water treatment agent which is not lowered in the performance during storage or distribution and can exert excellent scale inhibiting effect, in spite of its containing both phosphoric acid or a salt thereof and a natural deoxidizing agent; and a water treatment process. A boiler water treatment agent which contains not only a phosphoric acid component consisting of a condensed phosphoric acid and/or a salt thereof but also a natural deoxidizing agent containing a polyhydric phenol type organic compound and/or a polysaccharide type organic compound. Thewater treatment of a boiler water system is conducted by adding both the boiler water treatment agent and an alkali agent to the boiler water system.