PURPOSE: To improve the blocking characteristics and blow off residual liquid completely by a method wherein at the deepest spot of a recessed part which is provided on the lower surface of a valve piston, a plurality of injection holes from which air is injected are provided, and on the top of the valve piston, an auxiliary valve and a spring which energizes the auxiliary valve downward are provided.

CONSTITUTION: Asphalt emulsion which is pushed out from a fixed volume apparatus 8 flows into an inner tube 25 of an injector 11 through a connecting pipe 16. When a valve piston 31 is pulled up by an air cylinder 33 for opening/closing, an auxiliary valve 45 is also pulled up, and a fixed volume of the asphalt emulsion flows out from a nozzle 28 by a fixed pressure and injected in a paper bag 3. When the valve piston 31 is pushed down, the nozzle 28 is closed while the asphalt emulstion which adheres to the inner surface of a lower side part 28b is being scraped off. The auxiliary valve 45 engages with the inner peripheral surface of an intermediate part 28c, and is pressed by a coil spring 51, and the nozzle 28 is doubly closed by the valve piston 31 and auxiliary valve 45. When compressed air is sent into an air passage 42 of a valve rod 30, the air is injected from injection holes 43 along the conical surface of a recessed part 40, and residual asphalt is completely blown off and blown into the paper bag 3.