an embodiment of the present invention will be described referring to fig1 , 2 a , 2 b , 3 a and 3 b . fig1 is a plan view schematically showing a plug opening unit of a specimen preprocessing system . the plug opening unit 1 is connected to a carrier line 2 for introducing a specimen rack 4 through a specimen rack inlet 3 . the introduced specimen rack 4 is carried to a determination section 6 through a buffering section 5 which is formed to prevent the specimen rack 4 sequentially carried from the carrier line 2 from being stuck . a camera 7 is installed in the determination section 6 such that the determination is made through image processing . information with respect to the specimen container , for example , height , external diameter , and the plug configuration is preliminarily registered to be compared with the actually processed image of the specimen container so as to make the determination with respect to the height , external diameter and the plug configuration of the specimen container . as the specimen containers ( one to five ) mounted in each specimen rack may be subjected to an image processing simultaneously , the determination time may be markedly shorter than the time for the generally employed determination for each specimen container . identification numbers 13 a to 13 j , for example , bar codes are allocated to predetermined positions of the specimen racks 4 respectively to allow the determination with respect to the identification number of the rack simultaneously with the image processing . identification numbers 11 a to 11 j , for example , barcodes are also allocated to respective specimen containers 10 a to 10 j , which allow determination with respect to the identification numbers of the specimen containers through the image processing . the determination section determines with respect to the height and the external diameter of the specimen container , the plug configuration , the presence / absence of the plug , the identification number of the specimen container , and the identification of the specimen rack to determine with respect to the operation for opening the respective specimen containers . after performing the determination with respect to the height , the external diameter , the plug configuration , the presence / absence of the plug , the identification number of the specimen container , and the identification of the specimen rack , when the result becomes indeterminable , the error is reported to a host - computer , and the failure specimen rack or the failure specimen container will be carried to a plug opening unit feed section . then it will be stored in an error specimen rack storage section disposed in the specimen preprocessing system via the carrier line 2 . the plug opening operation for the specimen rack 4 that is determined as having been provided without any plug is skipped . when all the specimen containers 10 a to 10 j in the specimen rack 4 are not plugged , they are carried to the conveyor line 2 through a specimen rack outlet 9 and further to another specimen preprocessing unit . the specimen rack 4 determined as being normal is carried to a plug opening position 8 , and allows a specimen container lifting mechanism 14 to grip the specimen container at a predetermined position so as to be lifted to the plug opening position for opening the plug . upon completion of the plug opening operation , the specimen rack 4 is carried to the determination section to allow the camera 7 to process the image again . when the plug opening fails , the specimen rack is carried to the plug opening position 8 again by an arbitrary retry number of times for performing the plug opening operation . if the opening operation still fails in spite of the retry arbitrarily set number of times , the error is reported to the host - computer . the specimen rack determined as the error is stored in the error specimen rack storage section disposed in the specimen preprocessing system . if the opening operation ends successfully , the rack is carried to the carrier line 2 through the specimen rack outlet 9 of the plug opening unit and further to another specimen preprocessing unit . the flow of the processes of the specimen container lifting mechanism and the plug opening mechanism in the plug opening position 8 will be described referring to fig2 a and 2b . the specimen container lifting mechanism 14 has the stroke for lifting the specimen container 16 different from the one for lifting the specimen container 17 depending on the result of the determination section 6 . the position of the plug opening mechanism is always kept constant . the plug opening operation is simplified by making the lifting stroke variable . fig3 a and 3b show the state where the specimen containers 10 a to 10 j each having different height , external diameter and plug configuration are mounted in the specimen rack 4 . fig3 a shows plugs 12 a , 12 b and 12 c and fig3 b shows plugs 12 e - 12 i . the identification numbers 13 a , 13 b are allocated to predetermined positions of the specimen racks 4 , the identification numbers are also allocated to the respective specimen containers 10 a to 10 j . the determination section 6 performs the image processing in the aforementioned state to determine the identification numbers of the specimen rack 4 and the specimen containers , respectively , so as to determine with respect to the height , the plug configuration and the external diameter of the respective specimen containers . a height adjustment spacer 18 is disposed at the lower portion of the specimen container . the image processing may be performed to determine with respect to the spacer 18 so as to accurately determine with respect to the specimen containers each with similar height . if the specimen container has a special configuration such as the specimen container 10 f , the specimen container lifting mechanism 14 sets the grip position of the specimen container above the general level to avoid a plug opening operation failure owing to unstable grip at the tapered portion . it should be understood by those skilled in the art that various modifications , combinations , sub - combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof .