A fault detecting apparatus for a four-wheel drive vehicle is provided for avoiding without fail any erroneous detection due to a response delay associated with an operation for switching a speed increasing system and due to sudden fluctuations in wheel speed, to highly accurately detect a fault in the speed increasing system. The speed increasing system is configured to transmit a driving force of a prime mover directly to main driving wheels and to sub-driving wheels through associated clutches, and to increase rotational speed of the sub-driving wheels above rotational speed of the main driving wheels when a speed increase instruction signal is outputted. The fault detecting apparatus comprises an S-AWD.cndot.ECU for detecting a fault in the speed increasing system, and for disabling the fault detection until a predetermined time has elapsed after a speed increase instruction signal was outputted or after the output of the speed increase instruction signal was stopped.