A device for forming headrest in a foaming die, which includes a lid engagement mechanism operable to keep a lid member engaged in an injection nozzle placed in the foaming die, thereby preventing leakage of liquid foaming agent from the injection nozzle. Further, the device includes a lid disengagement mechanism operable to disengage the lid member from the injection nozzle and an injection gun movable vertically. The lid disengagement mechanism includes a pressure element provided at the injection gun. Upon downward movement of the injection gun, the pressure element acts on the lid member, so that the lid member is disengaged from the injection nozzle and the injection gun is then engaged with the injection nozzle, thereby allowing liquid foaming agent to be injected into a trim cover assembly placed in the foaming die.