An automatic retractable syringe includes a tubular body on which a syringe opening is provided; a piston body in whose pressing portion a vent hole VA is formed and in whose shaft portion a vent passage passing from the vent hole VA through inside to the front end portion is formed; and a spring which returns the pressed piston body to its original position. When the pressing portion is pressed by a thumb or other finger plugging the vent hole VA to move the piston body forward, air is supplied from the syringe opening. Subsequently, when the thumb or other finger is released, air flows into the tubular body through the vent hole VA and the vent passage, and the piston body automatically returns to its original position by the spring. Therefore, air can be consecutively supplied by repeatedly pressing the pressing portion.