PURPOSE: To obtain the same molded product as the surface state of the mold with a constant height by a method in which the heat exchangers at end parts are displaced by a pushing means so as to keep the level of a cell content at a specified heigth, while detecting said level when the content is at molten state, and the heat exchangers at end parts are pushed by the pushing force equal to or more than a constant value when said content approaches to a solid state, and this process is achieved by using a computer.

CONSTITUTION: Although the volume of the content in a cell changes in accordance with the progress of polymerization reaction, at the prior half of polymerization process where the cell content is fluid, the level of the cess content changing accordingly to the volume change of the cell content is deticted, and a pushing means 7 are operated accordingly to its result. The levels of the cell contents are kept at a specified height, changing the gap between the cells 6 by the pushing force generated by said means. This process is achieved by using a computor. When the content approaches to a solid state at polymerization-progress, instead of the moving operartion reducing the distance between the heat exchangers at both ends, the pushing force capable of correcting the deformation of each cell content is applied to the heat exchangers 1 at both ends, until the polymerization is finished.