The present invention provides a reactor for carrying out in a single container the steps of the extraction of nucleic acids and the amplification of a nucleic acid, the procedures of the steps being usually different to each other. The reactor according to the present invention is a reactor for detecting a target nucleic acid from a sample, comprising at least a first compartment which contains an extraction reagent composition for extracting nucleic acids from said sample, a second compartment which contains an amplification reagent composition for amplifying the target nucleic acid, a separating means for separating the first and second compartments, and an aperture which enables to introduce said sample into only said first compartment. The separating means breaks the separation of the first and second compartments by physical energy supplied from the outside of the reactor, and thereby makes it possible to mix the extraction reagent composition in the first compartment and the amplification reagent composition in the second compartment.