A method for an automatic analyzer to reduce a user's workload required when a reagent is placed therein, and thereby facilitating change of the operation mode of the automatic analyzer. The automatic analyzer including a first reagent storing unit that is capable of storing a plurality of reagent cassettes and a second reagent storing unit that is capable of storing a plurality of reagent cassettes. When the first reagent storing unit does not have empty space for a required reagent, and all reagents required for analysis operation for a selected operation set cannot be completely transferred to the first reagent storing unit, keeping the required reagent waiting in the second reagent storing unit. Further, when the first reagent storing unit is provided with empty space, the required reagent is transferred to the first reagent storing unit.