An image data conversion processing device including an issue unit, plural line storing units, and a generating unit for converting to a television signal image data stored in plural kinds of developing formats in a storing unit. The image data comprises plural lines, and the television signal comprises a predetermined number of lines. The issue unit issues a transmission instruction of image data to the storing unit in accordance with a period specified by a ratio of the number of lines of the image data to be converted and the number of lines of the television signal. The plural line storing units cyclically store the image data transmitted from the storing unit line by line on the basis of the transmission instruction of the issue unit. In synchronism with the horizontal synchronizing signal of the television signal, the generating unit multiplies the image data stored in the line storing unit by an interpolative coefficient corresponding to the developing format of the image data to be converted in the plural interpolative coefficients, which are beforehand set for the plural kinds of developing formats, thereby generating the television signal.