PURPOSE: To improve the accuracy of finishing in succeeding machining processes, and to prevent a flaw and fouling in cloth by movably fitting a ring-shaped suction chamber with a porous top plate and sucking and fixing a bag base body set onto a table by bringing the pressure of the ring-shaped suction chamber to negative pressure.

CONSTITUTION: Since a suction head 23 sucking upper cloth 1 is formed in a ring-shaped suction head on machining, cloth at a central section can be held under the state in which no crease is generated, and can be machined extremely easily. When upper cloth 1 is separated, the inclination θ of upper cloth 1 must be reduced for increasing the separation height of upper cloth 1 up to specified height, but said inclination θ is diminished by making the diameter of the suction head 23 smaller than that of a ring-shaped suction chamber 13 for sucking lower cloth in the method. When upper cloth 1 is sucked by the suction head 23 and said suction pipe 23 is lifted gradually, cloth can be separated up to predetermined height by elongation by the resiliency of upper cloth 1 and pulling-in from the periphery only of upper cloth 1.