An image data processing apparatus is provided which is adapted to perform an image processing process suitable for each of a plurality of image modes on the basis of an output gradation reference curve and computation data. Output image data are generated by processing inputted image data on the basis of an output gradation curve which is prepared for each image mode on the basis of an output gradation reference curve and computation data. Therefore, output gradation curves for respective image modes need not preliminarily be stored in storage means. For example, if two output gradation reference curves and 16 computation data are preliminarily stored in the storage means, the inputted image data can be subjected to a process corresponding to any of 32 image modes by using an output gradation reference curve and a computation data in combination.