Abstract: A brake assist control for generating a brake force greater than that of a normal time is performed. It is determined (102) that a brake pedal is operated at a high speed when a rate of change .DELTA.PM/C of a master cylinder pressure PM/C exceeds a first threshold value .alpha.. When the cause of the braking operation is an external disturbance, the rate of change .DELTA.PM/C the rate of change .DELTA.PM/C changes immediately thereafter. When a period during which the rate of change .DELTA.PM/C becomes less than a second threshold value .beta. is shorter than a predetermined period T0, it is determined that the cause of the braking operation is the external disturbance such as a rough road or a step and an execution of the brake assist control is prohibited (104, 108, 110, 112).