A material bead charging method using a novel foam forming device comprising forming chambers (13, 14) formed independently of a cavity (4) on the rear sides of a set of forming molds (2, 3) for forming a foam formed product, respectively, so that a service fluid such as air and steam can be fed into the cavity (4) through a first opening part (30) independently of the chambers (13, 14), the method comprising feeding material beads into the cavity (4) while a pressure inside the cavity (4) is maintained, by using the first opening part (30), at a specified negative pressure relative to a pressure inside a material tank, whereby a variation in charging density due to the omission of vent holes can be prevented while various kinds of problems caused by the forming of vent holes are basically solved. |