Sensors detect rotational speeds of respective vehicle wheels. A slip detection reference value is calculated from the detected rotational speeds of the vehicle wheels. Acceptable slip ranges for the respective vehicle wheels are determined in accordance with the calculated slip detection reference value. The rotational speeds of the vehicle wheels are compared with the corresponding acceptable slip ranges respectively. Slip controls of the respective vehicle wheels are performed when the rotational speeds of the vehicle wheels reside outside the corresponding acceptable slip ranges respectively. The highest of at least two of the rotational speeds of the vehicle wheels is selected. Differences between the respective rotational speeds of the vehicle wheels and the selected highest vehicle wheel rotational speed are calculated. The slip detection reference value is corrected in accordance with the respective calculated differences.