PURPOSE: To achieve good multi-color printing, by allowing the running speed of a printing belt to be always synchronous to the reference speed in the feed teeth of a sprocket.

CONSTITUTION: When the running speed V3 of a pringing belt 18 is apt to become slower than the reference speed of feed teeth 38 at the point P being the central part in the thickness of a film (V3<V2), the rotary speed of an auxiliary motor 40 is controlled to impart positive driving to a tension roll 14 and, contrarily, when the running speed V3 of the printing belt 18 is apt to become faster than the reference speed V2 (V3>V2), the rotary speed of the auxiliary belt 18 is controlled to impart negative driving to the tension roll 14. Proper dirving synchronous to the reference speed V2 of the feed teeth 38 at the point P is imparted to the printing belt 18. Therefore, the possibility of color irregularity or printing shift is not generated in multi-color printing.