A method of automatically isolating and purifying nucleic acid from a nucleic acid-containing specimen is provided, the method comprising: injecting a liquid into a cartridge for isolation and purification of a nucleic acid including at least two openings from one opening of the at least two openings, in which the cartridge includes a container having the at least two openings and containing a nucleic acid-adsorbent solid phase; passing the liquid through the nucleic acid-adsorbent solid phase by a pressure difference generated by a pressure generation means for generating a pressure difference between the inside and outside of the container; and discharging the liquid from the other opening of the container to the outside of the container by a pressure difference generated by the pressure generation means, wherein a pressure generated in the inside of the container by the pressure generation means is measured, a pressure change velocity and a pressure change acceleration are calculated on the basis of the value of the measured pressure, and the timing of completion of discharge of the liquid from the container is determined by use of a temporal change pattern of at least one of the measured pressure, the pressure change velocity and the pressure change acceleration.