disclosed herein is an automatic analyzer that is capable of reducing a user &# 39 ; s workload required when a reagent is placed , and thereby facilitating change of the operation mode of the automatic analyzer . an automatic analyzer comprises : a reaction unit including a plurality of vessels ; a first reagent storing unit that is capable of storing a plurality of reagent cassettes ; a sample dispenser for dispensing a sample into the reaction unit ; a reagent dispenser for dispensing a reagent corresponding to an analysis item from the first reagent storing unit into the reaction unit ; a second reagent storing unit that is capable of storing a plurality of reagent cassettes ; and reagent cassette transfer means that is capable of transferring a reagent between the first reagent storing unit and the second reagent storing unit , the automatic analyzer further comprising reagent selection means for selecting a reagent to be used for analysis operation based an operation set , wherein the operation set specifies a combination of an analysis item and the scheduled number of analyses corresponding to the analysis item .