To provide an image processing device, a method and its program, and a display device capable of properly performing gray-scale extension processing with low costs. An image processing device including a detection unit 11 that detects a gray-scale correction application area that is an area where a gray-scale variation should be smoothed in a pixel row of an input digital image signal by successively scanning a pixel row of the digital signal, and an extension correction unit 12 that performs gray-scale correction processing on a gray-scale correction application area detected by the detection unit 11, wherein in the detection unit 11, an output pixel row on which gray-scale correction processing was performed is fed back to and used as an input signal to the detection unit 11, and a gray-scale correction application area is detected by comparing an adjacent gray-scale difference of an input image signal with an adjacent gray-scale difference of the fed-back output pixel row.