In a pretreatment apparatus for transferring and mixing a specimen liquid and a reagent in a reagent reservoir, comprising containers for holding the specimen liquid and the reagent, flow paths connecting the containers in series, and a dialysis flow path including a dialysis membrane facing to the flow path, the mixing is brought about by transferring the liquid from the containers to the flow paths and a return by stoppage of transferring the liquid, and subsequently the liquid is made flow into the dialysis flow path.