An image processing device by which a user can operate a window level value without being conscious of a photometric interpretation of an image is provided. In the image processing device, the operation direction for increasing or decreasing the window level value is reversed according to the photometric interpretation of the input image, according to whether the window level operation direction is the right-and-left direction or the up-and-down direction, and according to whether or not to execute the grayscale inversion.