An automatic analyzer is free from limitations on layout of various mechanisms, and thus causing no bottlenecks, for example, in a space-saving design of the automatic analyzer. This invention includes a coaxial planar duplex arrangement of two dilution disks each with annularly disposed dilution cells, and the dilution disks A and B operate independently of each other. Various mechanisms (parent-sample sampling mechanism, diluent delivery mechanism, diluent/sample mixing mechanism, and diluted-sample sampling mechanism) used in a dilution process can each access the two dilution disks. The dilution process for a parent sample, executed on the dilution disks A and B, can be continuously conducted by providing a fixed delay in operational timing between the two dilution disks.