The present invention uses an injection apparatus comprising: an injector which includes a mounting part on which a syringe is mounted, a needle assembly having a plurality of needles, a motor and a screw for actuating a piston of the syringe forward and backward； and a main body for controlling the forward and backward actuation of the piston of the syringe and drug injection. The present invention is configured as follows: a vacuum suction pressure is formed in the needle assembly that is in contact with the skin； injection is performed so that a drug is injected through a needle after it is determined that the needle is in close contact with the skin by comparing a sensed vacuum suction pressure value with a preset vacuum suction pressure value； and if injection is completed, the needle assembly is separated from the skin.
