An automatic analyzer which assures uniformity in mixing effects regardless of sample quantity and test item and thus produces analysis results with high repeatability. The automatic analyzer includes a device for adding a conditioning liquid into a reaction chamber so that the quantity of liquid in the reaction chamber becomes a predetermined quantity prior to being mixed. The conditioning liquid may be a diluent or physiological saline as used for dilution of a sample or any other special liquid that adjusts the properties such as viscosity, surface tension, etc. of liquid to be mixed.