The present invention is to present a reagent preparing apparatus for preparing a reagent for processing a sample, by using a first liquid and a second liquid, comprising: a storage unit for storing liquid; and a constant amount liquid quantifying unit comprising a constant amount holding instrument which is used in common to hold a constant amount of the first liquid and to hold a constant amount of the second liquid, the constant amount liquid quantifying unit performing an operation of transferring the constant amount of the first liquid to the storage unit and performing an operation of transferring the constant amount of the second liquid to the storing unit.