A filling material which enables a user to visually confirm the state of the filling material while it is injected into a flexible container based on an X-ray image, and a method of confirming the injection state of the filling material. The filling material configured to be injected into the flexible container includes a first contrast substance that is flowable, and a second contrast substance that is mixed into the first contrast substance and is insoluble in the first contrast substance. The second contrast substance possesses higher contrast X-ray contrast properties than the X-ray contrast properties of the first contrast substance.
