A sample-processing system comprising: a plurality of functional modules (400a-c) each having a different function to operate as either an analyzer, a preprocessor, or any other device, and each different in processing capability; a plurality of buffer units (300a-c) each combined with any one of the functional modules (400a-c) to form a pair; and a sample rack conveyance block including a sample rack loader, a sample rack conveyance unit, and a sample rack storage section, the sample rack conveyance unit being formed to interconnect each functional module (400a-c) and each buffer unit (300a-c) paired therewith; wherein each buffer unit (300a-c) includes, in addition to independent slots serving as a buffer portion adapted to hold a plurality of sample racks, a rack transfer mechanism configured for transferring the sample racks bi-directionally between the buffer unit (300a-c) and the sample rack conveyance unit, and the buffer unit (300a-c) is further constructed to make a sample rack from the sample rack conveyance unit temporarily stand by in the buffer unit (300a-c) according to a particular operational status of the associated functional module (400a-c), and thus to avoid congestion in the sample rack conveyance unit such that rack transfer will be continued, characterised in that each buffer unit (300a-c) includes sample rack moving means (360,370) adapted to load and unload one of the sample racks into and from any one of the slots and in that each buffer unit (300a-c) comprises a rack conveyance section configured for conveying the sample racks bi-directionally between the buffer unit (300a-c) and the functional module (400a-c) paired with the buffer unit (300a-c) independently of the sample rack conveyance block. a plurality of functional modules (400a-c) each having a different function to operate as either an analyzer, a preprocessor, or any other device, and each different in processing capability; a plurality of buffer units (300a-c) each combined with any one of the functional modules (400a-c) to form a pair; and a sample rack conveyance block including a sample rack loader, a sample rack conveyance unit, and a sample rack storage section, the sample rack conveyance unit being formed to interconnect each functional module (400a-c) and each buffer unit (300a-c) paired therewith; wherein each buffer unit (300a-c) includes, in addition to independent slots serving as a buffer portion adapted to hold a plurality of sample racks, a rack transfer mechanism configured for transferring the sample racks bi-directionally between the buffer unit (300a-c) and the sample rack conveyance unit, and the buffer unit (300a-c) is further constructed to make a sample rack from the sample rack conveyance unit temporarily stand by in the buffer unit (300a-c) according to a particular operational status of the associated functional module (400a-c), and thus to avoid congestion in the sample rack conveyance unit such that rack transfer will be continued, characterised in that each buffer unit (300a-c) includes sample rack moving means (360,370) adapted to load and unload one of the sample racks into and from any one of the slots and in that each buffer unit (300a-c) comprises a rack conveyance section configured for conveying the sample racks bi-directionally between the buffer unit (300a-c) and the functional module (400a-c) paired with the buffer unit (300a-c) independently of the sample rack conveyance block.