PURPOSE: To change a thickness of a sheet continuously in the direction of output of a parison in the case where a plastic sheet is manufactured of the parison.

CONSTITUTION: An accumulator head 10 which is capable of changing width dimensions of a die slit 34 to be formed of a die 22 and a core 24 which is fitted in the die 22 and its inner diametral part by moving the core 24 in the vertical direction by a thickness adjusting hydraulic cylinder 28 and a cutting edge 36 which is provided on the lower part of the die slit 34 of the accumulator head 10 and can cut the parison 35 in the vertical direction are provided. Furthermore, the title device possesses an air cylinder 38 which can move the cutting edge 36 between a position where the parison 35 is cuttable and a position where the parison is incapable of cutting and a developing member 40 which is provided below the cutting edge 36 and possesses the conical upper part which can develop the parison 35 cut off by the cutting edge 36 into a sheetlike state. A guide roller 42 which can guide a sheet 41 is provided on the lower part of a position where the developing member 40 faces on the cutting edge 36.