An automatic analyzer comprising: a reaction disk (6; 35) with reaction cells (12; 36) arrayed thereon; a dilution disk (3; 16, 17) with dilution cells (8; 18) annularly arrayed thereon; a parent-sample sampling mechanism (7; 19) for pipetting each parent sample (1; 38) from a sample array storage unit (2; 37) into each dilution cell (8; 18); a diluent delivery mechanism (9; 20) for delivering a liquid diluent to the dilution cell (8; 18) having the parent sample (1; 38) pipetted thereinto; a mixing mechanism (10; 21) for mixing the pipetted parent sample (1; 38) and the diluent in the dilution cell (8; 18); a diluted-sample sampling mechanism (11; 22) for pipetting into one of the reaction cells (12; 36) a diluted sample created by mixing the parent sample (1; 38) and the diluent in the dilution cell (8; 18); and control means, characterized in that two dilution disks (3; 16, 17) are arranged such that the dilution disks (3; 16, 17) operate independently with each other, the two dilution disks (3; 16, 17) are arranged within a spatial range over which successive actions of each of the mechanisms (7, 9-11; 19-22) extend, the two dilution disks (3; 16, 17) are adapted to rotate concentrically about a common axis and are arranged in two rows, one forming an inner peripheral region and the other forming an outer peripheral region, and the control means is adapted to control a dilution process for the parent sample (1; 38) such that the successive actions of parent-sample sampling, diluent delivery, diluent/sample mixing, and diluted-sample sampling are sequentially conducted for one dilution cell (8; 18) on the dilution disks (3; 16, 17). a reaction disk (6; 35) with reaction cells (12; 36) arrayed thereon; a dilution disk (3; 16, 17) with dilution cells (8; 18) annularly arrayed thereon; a parent-sample sampling mechanism (7; 19) for pipetting each parent sample (1; 38) from a sample array storage unit (2; 37) into each dilution cell (8; 18); a diluent delivery mechanism (9; 20) for delivering a liquid diluent to the dilution cell (8; 18) having the parent sample (1; 38) pipetted thereinto; a mixing mechanism (10; 21) for mixing the pipetted parent sample (1; 38) and the diluent in the dilution cell (8; 18); a diluted-sample sampling mechanism (11; 22) for pipetting into one of the reaction cells (12; 36) a diluted sample created by mixing the parent sample (1; 38) and the diluent in the dilution cell (8; 18); and control means, characterized in that two dilution disks (3; 16, 17) are arranged such that the dilution disks (3; 16, 17) operate independently with each other, the two dilution disks (3; 16, 17) are arranged within a spatial range over which successive actions of each of the mechanisms (7, 9-11; 19-22) extend, the two dilution disks (3; 16, 17) are adapted to rotate concentrically about a common axis and are arranged in two rows, one forming an inner peripheral region and the other forming an outer peripheral region, and the control means is adapted to control a dilution process for the parent sample (1; 38) such that the successive actions of parent-sample sampling, diluent delivery, diluent/sample mixing, and diluted-sample sampling are sequentially conducted for one dilution cell (8; 18) on the dilution disks (3; 16, 17).