A reagent preparing device for preparing a reagent to be supplied to a measurement device for measuring a specimen, comprising: a first liquid storage unit for storing a first liquid; a reagent storage unit for storing a prepared reagent, including the first liquid and a second liquid different from the first liquid; a first liquid discarding unit for discarding the first liquid stored in the first liquid storage unit; and a controller configured for measuring an accumulated time of the first liquid in the first liquid storage unit; and controlling the first liquid discarding unit to discard the first liquid stored in the first liquid storage unit when the accumulated time reaches a predetermined time is disclosed. A specimen measuring device and a reagent preparing method are also disclosed.