An automatic sample injector for injecting a liquid sample into a vaporization chamber of a gas chromatograph includes a syringe, a plunger adapted to move inside the barrel of this syringe, a stepping motor having a plurality of magnetic poles for moving the plunger up and down inside the barrel, a power source for supplying excitation currents to these magnetic poles, switches for these magnetic poles, and a control unit for controlling the switches or the power source such that the excitation current to one of a pair of the magnetic poles corresponding to a rotary angular position is increased and the excitation current to the other of the pair is simultaneously decreased.