PURPOSE: The pressure at the boiling part is controlled to the prescribed level of negative pressure and the boiling point of the treating water is changed according to the controlled pressure to effect uniform aging of cocoons at an appropriate temperature.

CONSTITUTION: The dipping tank 6, steaming chamber 7, steeping tank 8, boiling chamber 9 and the controlling tank 10 are set in order along the way of the endless chain 5 provided with a plurality of cocoon vessels at a proper interval. When the dipping tank 6 and the steeping tank 8 are filled with hot water of about 70°C, they play a role of sealing the steam in the steaming chamber 7. The cocoons pretreated in these devices are transferred to the boiling chamber 9 which is on the top part of almost U-shaped closed tank. A V-shaped tube water tank is connected to the inlet and another V-shaped tube for controlling tank 10 follows the outlet. A known pressure-reduction device 12 is set in the boiling chamber 9 to set the pressure to negative, lower than the atmospheric one. Cocoons are boiled in the boiling and controlling chambers 9, 10 according to the negative pressure. The air inside the cocoon is replaced whith steam and the pressure gradually increased to the atmospheric one in the controlling chamber 10, thus a sufficient amount of hat water comes into the inside of the cocoon.