An image data processing method for estimating an ink consumption amount includes: an image data compression step of compressing image data after RIP processing by run-length encoding; and an ink consumption amount estimation step of calculating a predicted ink consumption amount when printing based on image data is performed by an inkjet printing device based on information on the number of occurrences for each gradation value obtained when run-length encoding is applied to image data and information on an ink consumption amount for each gradation value held in an ink consumption amount table prepared in advance.