In the step (I), the period (minute(s)) for dispersing the carbon black species A showing a nitrogen adsorption specific surface area (N2SA-(A)value) of 130 m2/g or less in the dispersing solvent and that minute(s)) for dispersing the carbon black species B showing an N2SA-(B) value lower than the N2SA-(A) value by 25 m2/g or more in the dispersing solvent toy α(A) and α(B), respectively, and further representing the rotation number (rpm) of a rotor of a dispersing machine used in the dispersing when the carbon black species A is dispersed, and that (rpm) of a rotor of a dispersing machine used in the dispersing when the carbon black species B is dispersed by β(A) and β(B), respectively, the following expression is satisfied: 1.1α(B)×β(B)≦α(A)×β(A)≦1.5α(B)×β(B)   (1)