An image processing apparatus having an adding unit to add information by changing a data value of image data and a detecting unit to detect the information added to the image data. In a digital watermark image processing method in the image processing apparatus, filter passage image data is formed by performing an image filtering process to original image data, watermark core image data is formed by performing a process to obtain a difference between the filter passage image data and the original image data, change amount image data is formed by performing a process to obtain a value to change the data value of the image data on the basis of the watermark core image data and desired information to be added, and watermark inserted image data is formed by performing a process to add the original image data and the change amount image data.