In the present invention, the conditions necessary for the change from the reducing-pressure mode to the holding pressure mode are changed based on the length of the period of instability from the beginning of wheel slippage to the stopping of this slippage, and the length of the period of continued severe locking in which the slip ratio of the wheel exceeds a certain fixed value. Therefore, even when there is a disturbance in the form of roughnesses in the road surface or the like, this is determined to be a temporary phenomenon and the switching of the setting from the reducing-pressure mode to the holding pressure mode can take place in a reliable manner, and the usual, stable braking power can be achieved in correspondence with the relationship between the road surface and the tires.