A reagent preparing device capable of supplying a predetermined reagent, which includes a first liquid and a second liquid different from the first liquid, to a measurement section for measuring a specimen using the reagent, comprising: a pressure generator for generating pressure to transfer liquid; a reagent preparing section for executing a preparation operation of the predetermined reagent using the pressure generated by the pressure generator; and a controller configured for performing operations comprising: determining whether or not the reagent preparing section is executing the preparation operation, and controlling the generation of the pressure by the pressure generator according to the determination result, is disclosed. A reagent preparing method and a specimen processing system are also disclosed.