PURPOSE: To prevent vehicle wheels from being placed in their locked state by providing an unconditional self-diagnostic means which performs the self- diagnosis of an anti-skid control device regardless of whether at the braking or not, when the vehicle speed reaches a given preset value.

CONSTITUTION: Anti-skid control is performed by implementing a program memorized beforehand in a memory device based on wheel speed detection values WFL to WRR from wheel speed sensors 11FL to 11RR. Concretely, a simulated vehicle speed Vref is calculated through wheel speed detection values of WFL to WRR. For instance, the difference between the simulated vehicle speed Vref and each of wheel speed detection values WFL to WRR is used for calculating each wheel speed which comes in within a given slip ratio. The braking power to each wheel is controlled by increasing or decreasing an operating liquid pressure to each of wheel cylinder 10FL to 10RR so as to achieve this wheel speed. Thus, edge wheel is securely prevented from being locked while sufficient deceleration speed is obtained.