PURPOSE: To determine the precise potential of the adhesion and its momentary value with a simple configuration by use of a known measured value by using at least one fuzzy logic from variables of vehicle acceleration, wheel acceleration and control pressure.

CONSTITUTION: A fuzzy logic part 100 determines and outputs the potential μPot of coefficient of traction by use of corrected vehicle acceleration aX', corrected wheel acceleration aR', and braking pressure (p) which are inputs. Since some fuzzy rules have different results depending on whether the value of the potentials μPot of the adhesion coefficient is small or large, the fuzzy rule is weighted according to the area where the value of the potential μPot of the adhesion coefficient is just present. The weighting is performed by use of a classification block 101 and a correction block 102 through the control input of the fuzzy logic part 100. The classification block 101 detects whether the potential μPot of the adhesion coefficient is small, middle or large, and transmits the result to the correction block 102.