A needleless injector which can selectively induce the immune response and a method for introducing DNA including a region coding for an antigen to selectively produce an antibody in a living body of a mammal by using the needleless injector are provided. Disclosed is a needleless injector for injecting a DNA solution into an injection target area without using any injection needles. The needleless injector includes an accommodating unit which accommodates the DNA solution, a predetermined ignition device; and a predetermined nozzle unit. A temperature of the predetermined combustion product, which is provided during the pressurization, changes to a neighborhood of ordinary temperature within 20 msec after a pressure, which is applied to the DNA solution on account of the combustion of the igniter powder, reaches an initial peak discharge force during a pressurization process for discharging the DNA solution.