A foaming die device is provided, in which can be directly placed a basic armrest unit having a shaft element and a connecting bracket rotatably secured to the shaft element. The foaming die device comprises a lower die, an upper die, and an auxiliary die element of split type to be incorporated in a working die area defined by the lower and upper dies. The auxiliary die element comprises two die pieces that can be mated together so as to be in a slidable engagement about one end portion of the shaft element. Hence, the mated two die pieces or auxiliary die element is slidable along the shaft element for its partial engagement in a through-hole of the basic armrest unit, whereupon all the basic armrest unit and auxiliary die element may be directly placed in the working die area of lower and upper dies and subjected to foaming process.