An automatic analyzer comprising: a sample array storage unit including a sample disk on which parent sample containers each for accommodating a parent sample are arrayed; a reaction disk with reaction cells arrayed thereon; a dilution disk with dilution cells annularly arrayed thereon; a parent-sample sampling mechanism for pipetting each parent sample from the sample array storage unit into each dilution cell; a diluent delivery mechanism for delivering a liquid diluent to the dilution cell having the parent sample pipetted thereinto; a mixing mechanism for mixing the pipetted parent sample and the diluent; a diluted-sample sampling mechanism for pipetting into one of the reaction cells a diluted sample created by mixing the parent sample and the diluent in the dilution cell; and control means that controls the operation of each kind of mechanism as well as operation of each kind of disk; characterised in that the control means is adapted to control a dilution process for the parent sample such that successive actions of parent-sample sampling, diluent delivery, diluent/sample mixing, and diluted-sample sampling are sequentially conducted for one arbitrarily selected dilution cell on the dilution disk; the control means is adapted to control the access operations of the parent-sample sampling, the diluent delivery, the diluent/sample mixing, and the diluted-sample sampling mechanism such that only one mechanism is allowed to operate with a dilution cell of the dilution disk while the rotation of the dilution disk is interrupted; and the access positions of the parent-sample sampling, the diluent delivery, the diluent/sample mixing, and the diluted-sample sampling mechanism are freely determined with respect to the dilution disk. a sample array storage unit including a sample disk on which parent sample containers each for accommodating a parent sample are arrayed; a reaction disk with reaction cells arrayed thereon; a dilution disk with dilution cells annularly arrayed thereon; a parent-sample sampling mechanism for pipetting each parent sample from the sample array storage unit into each dilution cell; a diluent delivery mechanism for delivering a liquid diluent to the dilution cell having the parent sample pipetted thereinto; a mixing mechanism for mixing the pipetted parent sample and the diluent; a diluted-sample sampling mechanism for pipetting into one of the reaction cells a diluted sample created by mixing the parent sample and the diluent in the dilution cell; and control means that controls the operation of each kind of mechanism as well as operation of each kind of disk; characterised in that the control means is adapted to control a dilution process for the parent sample such that successive actions of parent-sample sampling, diluent delivery, diluent/sample mixing, and diluted-sample sampling are sequentially conducted for one arbitrarily selected dilution cell on the dilution disk; the control means is adapted to control the access operations of the parent-sample sampling, the diluent delivery, the diluent/sample mixing, and the diluted-sample sampling mechanism such that only one mechanism is allowed to operate with a dilution cell of the dilution disk while the rotation of the dilution disk is interrupted; and the access positions of the parent-sample sampling, the diluent delivery, the diluent/sample mixing, and the diluted-sample sampling mechanism are freely determined with respect to the dilution disk.