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 reagent preparing section for preparing the predetermined reagent; a characteristic measurement device for measuring characteristic of the reagent prepared by the reagent preparing section; and a controller configured for performing operations comprising, controlling the supply of reagent prepared by the reagent preparing section to the measurement section according to the measurement result by the characteristic measurement device, and calibrating the characteristic measurement device based on a known characteristic value of a standard liquid having the known characteristic value and a measurement result obtained by measuring the characteristic of the standard liquid by the characteristic measurement device, is disclosed. A specimen processing system is also disclosed.