In an apparatus for detecting a skid occurred on a four-wheel drive vehicle having a prime mover and a transmission, an actual rear right/left wheel speed ratio between the rear right/left wheel speeds is calculated based on detected front and rear wheel rotational speeds, a front/rear wheel slip ratio during grip-driving is calculated by retrieving characteristics of a front/rear wheel slip ratio set with respect to a rear right/left wheel speed ratio using the calculated actual rear right/left wheel speed ratio, an actual front/rear wheel slip ratio is calculated based on the detected front wheel rotational speed and the rear wheel rotational speed, and occurrence of skid is then determined based on a difference between the calculated front/rear wheel slip ratio during grip-driving and the calculated actual front/rear wheel slip ratio.