A signal processing method includes a first step of calculating a value indicating a value obtained by multiplying a ratio of the number of times of inputting the input signal having any one of values from p to m, where m is a maximum value of values of input signal which are subject to said signal processing and p is a value smaller than m and not a minimum value of the input signal, within a predetermined period to the number of times of inputting the input signal within the predetermined period, by the variable range of the converted value; and a second step of subtracting the calculated value from a maximum value within the variable range of the converted value or a value near the maximum value, wherein the input signal is converted according to the conversion characteristic specified based on a value obtained by subtraction.