To provide an automatic analyzer, a reagent container replenishing apparatus and a reagent auto-loading method for improving an analysis efficiency by shortening the reagent container replacement time.

The automatic analyzer 1 includes, reagent storages 2, 3 for storing and refrigerating reagent containers 30 containing reagents, a reserve reagent storage 21 for storing and refrigerating the reagent containers 30 containing the reagents for replenishment and replacement, and a delivery device 22 for delivering the reagent container 30 for replacement stored in the reserve reagent storage 21 to the reagent storages 2, 3 and for delivering the empty reagent container 30 from the reagent storages 2, 3 to the reserve reagent storage 21. In the automatic analyzer, the reserve reagent storage 21 is placed adjacent to the reagent storages 2, 3 and has a cover part 21b having an opening/closing part for taking the reagent containers 30 in and out of the reserve reagent storage 21, and the opening/closing part is a part of the external cover of the automatic analyzer 1 and enables the reagent containers 30 to be taken in and/or out from outside of the automatic analyzer 1 while the automatic analyzer is in operation.