PURPOSE: To remove instability due to individual speed changing characteristics of motors and permit centralized speed change in a short period of time, by a method wherein the speeds of a motor for extrusion, a motor for blowing and a motor for a pulling machine are permitted to be controlled by a speed change curve, having arbitrary speed changing rate with respect to a same changing rate command.

CONSTITUTION: The speeds of a motor 1a for extrusion, a motor 4a for blowing and a motor 8 for a pulling machine are controlled by individual motor controllers respectively while the motor controllers are driven and controlled centralizedly by a system control unit 9. The setting of a speed change curve and speed change starting timing other than the setting of starting condition can be effected by operating ten-key pad 22 and command key 23 while observing a picture in a CRT display 21 attached to the front surface of the system control unit 9 while the command key 23 carry out a previously incorporated base sequence. Upon changing the kind of works, the speeds of respective motors 1a, 4a, 8 are controlled so as to be synchronized in the maximum followable time, operated by a speed change rate, preset in a speed parameter setting table, and the width of variation of the rotation of motors.