A signal processing method includes: inputting a first image signal and a second image signal, the first image signal containing gradation levels of respective pixels of a first frame, the second image signal containing gradation levels of respective pixels of a second frame that is continuous from the first frame; storing at least one predetermined gradation level as a correction-free gradation level that needs no correction; determining whether or not the gradation level of the first frame for every pixel is the stored correction-free gradation level, based on the input first image signal; and outputting a gradation level in the input second image signal by using, as a correction-free pixel that needs no correction, a pixel for which the gradation level of the first frame is determined as the correction-free gradation level.