PURPOSE: To execute the whole die changing operation at a high speed by adjusting the air pressure in a buffer tank to a preload, and communicating a cushion cylinder with a buffer tank to quickly secure a die cushion pressure.

CONSTITUTION: A normal correction factor containing the ratio of the content volume of the cushion cylinder 11 to the content volume of the buffer tank 20 is predetermined. The die cushion pressure P to be secured is set in the die cushion cylinder 11. A pre-load PT2 to be secured in the buffer tank 20 is required from this set die cushion pressure and the correction coefficient. Then, the air pressure in the buffer tank 20 is adjusted to be controlled to the pre-load PT2. Thereafter, the cushion cylinder 11 and the buffer tank 20 are communicated with each other to secure the die cushion pressure P quickly. In this way, the die cushion pressure can be controlled automatically, quickly and securely.