Injection molding method

An injection molding method using a die with a cavity into which molten resin is to be injected to fill the cavity from an extrusion barrel, the die having a shape that causes the molten resin injected into the cavity to branch and merge, the injection molding method comprising:

• providing bubbles into the molten resin by injecting a gas with a predetermined volume into a first passage through which the molten resin flows from a second passage, wherein the first passage communicates the extrusion barrel with the cavity and the second passage is connected to the first passage at a first point;
• generating a set of bubbles by breaking up the bubbles, wherein the breaking up is performed by applying first ultrasonic waves to the molten resin in the first passage, at a location closer to the cavity than the first point;
• injecting the molten resin containing the set of bubbles from the first passage into the cavity;
• and
• bursting the set of bubbles contained near meeting surfaces of the injected molten resin in a merging location in the cavity where the molten resin merges by applying second ultrasonic waves to the meeting surfaces of the injected molten resin, wherein the application of second ultrasonic waves starts before the molten resin merges in the merging location.