An automatic purging method for an injection molding machine which is capable of injecting an arbitrary material among different kinds of materials into a mold without requiring a resin replacing block or a purging container and an additional material supply means for increasing the material supply pressure, and which has a nozzle valve in a nozzle unit. This method is used so as to enable the automatic purging to be done smoothly and speedily by operating the nozzle valve so that it behaves according to a predetermined program. This automatic purging method consists of the steps of supplying a fresh material from a hopper (7) into a screw cylinder (1), rotating a screw with a nozzle valve (4) closed, for a predetermined period of time or until the screw has been moved back a preset stroke, to plasticize the resin, and then purging the molten resin with the nozzle valve (4) set to a predetermined degree of opening.