Abstract: A method for diluting and mixing specimen such as blood and a reagent, which is designed to prepare a mixed solution of a specimen and a reagent at a specific ratio by discharging the reagent in a passage for quantitative determination in a sampling valve by a specified amount of the reagent. The method is such that specimen is discharged by reagent first at a low speed and thereafter at a usual speed by a reagent discharge device. The reagent discharge device may comprise a syringe, a pump having a thin film for forming air and liquid chambers, or the like. As constructed, the specimen is not damaged due to fluid pressure.