A reagent preparing device for preparing a reagent to be supplied to a measurement section for measuring a specimen; the reagent preparing device comprising: a reagent preparing section for preparing a reagent including a first liquid and a second liquid, different from the first liquid; a reagent storage container, connected to the measurement section, for storing the reagent prepared by the reagent preparing section; and a controller configured for determining whether or not the reagent stored in the reagent storage container is suppliable to the measurement section based on reagent expiration date information related to an expiration date of the reagent stored in the reagent storage container, is disclosed. A reagent preparing method and a specimen processing system are also disclosed.