A sequencer pretreatment method using a sequencer pretreatment device comprising: a suction and discharge mechanism configured to suck and discharge gas; a nozzle head having one or more nozzles configured to detachably mount one or more dispensing tips communicated with the suction and discharge mechanism and capable of sucking and discharging liquid; a container group comprising at least a reaction vessel and a liquid accommodating portion for accommodating various liquids including magnetic particle suspension; a moving mechanism configured to cause relative movement among the nozzle and the container group; and a magnetic unit capable of exerting a magnetic field to the inside of the dispensing tip mounted to the nozzle, the method comprising: an extraction step of mixing a sample, extraction reagent solution, and magnetic particle suspension accommodated in the container group and extracting nucleic acid from the sample; characterised by: a fragmentation producing step of fragmentating the extracted nucleic acid by mixing with fragmentation solution accommodated in the container group, mixing at least the fragmentated nucleic acid, the magnetic particle suspension, and a binding promoter accommodated in the container group, allowing a fragment to bind with a magnetic particle, and producing a fragment of base sequence having the number of bases within a predetermined range according to a sequencer, wherein this range is set by a user, by adding dissociation solution accommodated in the container group such that a desired concentration of the binding promoter is obtained based on a relationship between a concentration of the binding promoter and the number of bases in a fragment of base sequence, and causing the fragment to be dissociated from the magnetic particle using the sequencer pretreatment device; and an amplification pretreatment step of dispensing a predetermined volume of solution containing the produced fragment into the reaction vessel together with amplification solution using the sequencer pretreatment device. a suction and discharge mechanism configured to suck and discharge gas; a nozzle head having one or more nozzles configured to detachably mount one or more dispensing tips communicated with the suction and discharge mechanism and capable of sucking and discharging liquid; a container group comprising at least a reaction vessel and a liquid accommodating portion for accommodating various liquids including magnetic particle suspension; a moving mechanism configured to cause relative movement among the nozzle and the container group; and a magnetic unit capable of exerting a magnetic field to the inside of the dispensing tip mounted to the nozzle, the method comprising: an extraction step of mixing a sample, extraction reagent solution, and magnetic particle suspension accommodated in the container group and extracting nucleic acid from the sample; characterised by: a fragmentation producing step of fragmentating the extracted nucleic acid by mixing with fragmentation solution accommodated in the container group, mixing at least the fragmentated nucleic acid, the magnetic particle suspension, and a binding promoter accommodated in the container group, allowing a fragment to bind with a magnetic particle, and producing a fragment of base sequence having the number of bases within a predetermined range according to a sequencer, wherein this range is set by a user, by adding dissociation solution accommodated in the container group such that a desired concentration of the binding promoter is obtained based on a relationship between a concentration of the binding promoter and the number of bases in a fragment of base sequence, and causing the fragment to be dissociated from the magnetic particle using the sequencer pretreatment device; and an amplification pretreatment step of dispensing a predetermined volume of solution containing the produced fragment into the reaction vessel together with amplification solution using the sequencer pretreatment device. a fragmentation producing step of fragmentating the extracted nucleic acid by mixing with fragmentation solution accommodated in the container group, mixing at least the fragmentated nucleic acid, the magnetic particle suspension, and a binding promoter accommodated in the container group, allowing a fragment to bind with a magnetic particle, and producing a fragment of base sequence having the number of bases within a predetermined range according to a sequencer, wherein this range is set by a user, by adding dissociation solution accommodated in the container group such that a desired concentration of the binding promoter is obtained based on a relationship between a concentration of the binding promoter and the number of bases in a fragment of base sequence, and causing the fragment to be dissociated from the magnetic particle using the sequencer pretreatment device; and an amplification pretreatment step of dispensing a predetermined volume of solution containing the produced fragment into the reaction vessel together with amplification solution using the sequencer pretreatment device.