The ammonia aqueous solution manufacturing apparatus manufactures a lean ammonia aqueous solution by supplying and dissolving ammonia from the ammonia supply apparatus to ultrapure water supplied from an ultrapure water production apparatus, and supplies this aqueous ammonia solution to a use point. Here, the ultrapure water producing system has an ability of supplying ultrapure water (W) having a resistance value of 18 MΩ · cm or more and a metal ion concentration of 1 ng / L or less, particularly 0.1 ng / L or less. Then, ammonia is added to the ultrapure water W from the ammonia supply device to prepare lean ammonia water. This ammonia aqueous solution manufacturing apparatus is suitable for manufacture of lean ammonia water whose density | concentration of ammonia (ammonium ion) is 100 mg / L or less, especially 50 mg / L or less. Such ammonia aqueous solution manufacturing apparatus can manufacture ammonia aqueous solution at a stable density | concentration, and is excellent in the followability to a change of density | concentration.
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