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 needle； the needleless injector comprising an accommodating unit which accommodates the DNA solution； a predetermined ignition device； and a nozzle unit having a discharge port through which the DNA solution pressurized by the combustion of the predetermined igniter powder in the ignition device flows so that the DNA solution is discharged to the injection target area； wherein 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.
