A reagent preparation apparatus for supplying a reagent, prepared by mixing a high concentration reagent and purified water supplied from a purified water production device, to a sample measurement device for measuring a sample using the prepared reagent, the reagent preparation apparatus comprising: a reagent preparation unit for preparing the reagent by mixing the high concentration reagent and the purified water supplied from the purified water production device; and a controller, communicably connected to the purified water production device, for controlling the purified water production device to continue producing the purified water until a predetermined time has elapsed from when the purified water production device starts the production of the purified water.Also, a reagent preparation system and a reagent preparation method.