To provide a reactor enhancing work efficiency when injecting a reagent into a flow passage.

This reactor includes: a device 3 arranged in its inside with the flow passages 1, 2 through which a specimen is moved; a large number of magnetic beads stored inside the sub-flow passage 2 and having affinity to the specimen; an electromagnet 5 arranged under the condition faced to the flow passages 1, 2 in an outside of the device 3; and a moving means for moving the electromagnet 5 along the flow passages. The reactor is provided with: a narrow passage 10 extended from the sub-flow passage 2 to an outside of a moving range of the electromagnet 5; and an injection part 11 connected to the sub-flow passage 2, and opened in a rear face opposed to the electromagnet 5 in the device 3. A passage cross-sectional area of the narrow passage 10 is narrower than a cross-sectional area of the sub-flow passage 2, and an opening area of the injection part 11 is wider than the passage cross-sectional area of the narrow passage 10.