To provide a pH adjusting method of a boiler water system and a pH adjuster thereof by which the pH of the boiler water system can be adjusted by using a material, which is distributed in a large amount, is easy to obtain and has little influence on a human body, and also by which the problems of handleability in use and an odor can be improved.

The pH adjusting method for adjusting the pH of the boiler water system 41 using an aqueous ammonia solution includes a process of introducing a raw material aqueous solution 15 in which ammonium salt is dissolved to an anion exchange column 11a for the raw material aqueous solution, removing at least a part of the counter ions of ammonium ions and communicating the anion exchange column 11a for the raw material aqueous solution with a predetermined part (water supply system) 42 and/or predetermined part (condensate system) 43 of the boiler water system 41, and treated liquid obtained when the raw material aqueous solution 15 passes through the anion exchange column 11a for the raw material aqueous solution is supplied through communication pipes L3, L3a and L3b to the predetermined part (water supply system) 42 and/or the predetermined part (condensate system) 43.