An automatic infinitesimal liquid reactor equipped with a pipetter for dispensing reagents employs two stages which are capable of sliding horizontally. A microplate which has a plurality of recesses in rows is placed in one stage and reagent bottles are placed on the other stage. The pipetter is supported on a horizontal guide rail operating slidably horizontally and along an axis perpendicular to the direction in which the two stages slide. The horizontal guide rail is supported on a vertical guide rail so as to operate slidably in a vertical direction. Thus the pipetter can be moved in a plane perpendicular to the direction in which the two stages slide. When reagents are sucked from reagent bottles, the second stage is slid so that a reagent bottle is superposed below the pipetter and the pipetter is moved downwardly into the reagent bottle to draw in a reagent. When the reagent is discharged, the first stage is slid so that the microplate is superposed below the pipetter and the pipetter is moved downwardly into a recess to discharge the reagent, thereby dispensing a reagent efficiently and minimizing the space required for the reactor.