An image signal processing device 1 comprises an encoding part 10, a delay part 20, a difference calculation part 30, a decoding part 40, a quantization error determination part 50 and a corrected image data output part 60. The corrected image data output part 60 outputs image data G2 input to the encoding part 10 as corrected image data G2′ when a difference D output from the difference calculation part 30 is equal to or less than a predetermined value Dth, or determines a basic correction value from the image data G2 input to the encoding part 10 and decoded image data GB1 output from the decoding part 40, reduces the basic correction value by a value in accordance with the magnitude of a quantization error E determined by the quantization error determination part 50 to obtain a modified correction value, and outputs the image data G2 input to the encoding part 10, to which the modified correction value is added, as the corrected image data G2′ when the difference D is greater than the predetermined value Dth.