The present invention provides an automatic analyzer for analyzing blood, urine, or other samples in which, for the purpose of reducing the workload of the operator, the process of reagent replacement is automated such that the measurement operation of the analyzer need not be halted during the reagent replacement. A sample rack transfer mechanism and reagent storage/transfer mechanisms are placed on different levels of two-story structure. This configuration allows reagent replacement even when the analyzer is performing an analysis, as well as achieving size reduction of the analyzer. The automation of reagent replacement is achieved by the following: a sensor's detection of the remaining reagent amount in a reagent vessel reaching a particular value; and the analyzer's detection of a measurement request for a sample rack when necessary reagents are not present in any reagent storage mechanism of an analyzer unit.