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 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 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. <EOS>