PURPOSE: To prevent possible slackening of a sheet during conveyance by temporarily storing it with a moving frame between a extruder for continuously extruding the sheet and an intermittently working molding machine.

CONSTITUTION: While a molding machine 7 is under the die clamping, a moving frame 6 initially positioned as indicated by the two-dot chain line is moved to the left and an auxiliary moving frame 11 downward so that a sheet M extruded continuously from an extruder 1 is inserted between a sheet guide belt 8 and a sheet conveying device 9 for temporary storage. Then, when upper and lower dies 14 and 15 of molding machine 7 are opened, a mobile belt conveyor 20 is moved to the left to retain a molding m (n) with a clip 21. As soon as the conveyor 20 is shifted to the right, the frame 6 is moved to the right (the frame 11 is moved upward) to carry the sheet M into the molding machine 7. Thus, the operation is shifted to the subsequent molding cycle.