PURPOSE: To measure the degree of cure of a synthetic resin film during the formation of the film by measuring the total light transmissivity of the film while the film is run during the forming process of the film.

CONSTITUTION: A raw material is put in a hopper 3 through a stirring and mixing and filtrating tanks 1 and 2 and degassed. The degassed raw material is brought into contact with a first and second casting rolls 4 and 5 through a polyethylene terephthalate (PET) film 6 used as a carrier having excellent smoothness and release characteristics in order to obtain a smooth synthetic resin film. The film 6 is wound around a take-up body 8 and a formed vinyl chloride resin film passed through a total light transmissivity measuring instrument 12 is wound around another take-up body 11. During the formation of the film 4, the temperature of the roll 4 is fixed and that of the roll 5 is set in seven stages and the data of the measuring instrument 12 are read. The degree of cure of the film 4 is recognized by comparing the data of the instrument 12 with already set data by factors, such as the kind, thickness, forming speed, etc., of the raw material of the film 4. Therefore, the degree of cure of the film 4 can be measured in the forming process of the film 4 without destroying the product of the film 4.