To accurately determine a sudden change in the coefficient of friction of a drive road surface from a low level to a higher one even while either of wheel brakes for a front wheel and a rear wheel installed in a motorcycle is operated independently.

A first condition is set that during antilock control of both wheel brakes, both wheel brakes be in a state wherein a pressure increase in a current cycle is larger than the product of a pressure increase in the preceding cycle multiplied by a first set multiple larger than one, and a second condition is set that during antilock control of either of the two wheel brakes, either of the two wheel brakes be in a state wherein the pressure increase in the current cycle is larger than the product of the pressure increase in the preceding cycle multiplied by a second set multiple larger than the first set multiple. If either of the first and second conditions is met, a sudden change is determined in the coefficient of friction of a drive road surface from a low coefficient of friction to a higher coefficient of friction.