A hollow poppet valve has an internal cavity which extends from within its valve head into a valve stem portion and is partly filled with a coolant. A manufacturing method includes forming a coolant rod by cutting clayish coolant to a coolant rod of a predetermined length from the coolant material extruded from an extruder; a coolant-rod insertion step in which a coolant rod is inserted into a cavity formed in an intermediate valve product through its opening; and a step of sealing the opening of the hole. In the coolant-rod insertion step, coolant-rods are inserted into the cavity separately in multiple times. Since each coolant rod is short, it is little deformed or bent, so that it has little interference with the inner periphery of the cavity, which permits use of a thick rod and increase the amount of the coolant in the cavity.