A signal processing method includes addition of noise obtained by multiplying generated random number by K to the input pixel signal I(x), a binarization process of comparing the result of the addition with two thresholds, and a process of calculating a probability. The binarization process includes a first nonlinear process and a second nonlinear process. The first nonlinear process outputs “P” in a case where I(x) after the addition of the noise is greater than the threshold T1 and less than the second threshold T2. The second nonlinear determines “1” or “0” for a processing target pixel, in which the result of the first nonlinear process is “P,” based on input pixel signals of pixels around the processing target pixel. The process of calculating a probability calculates a probability J(x) that the result of the first nonlinear process is “1,” or the result of the first nonlinear process is “P” and the result of the second nonlinear process is “1”.