This invention is directed to accurately specifying the spot where abnormality has occurred in the speed change system of a vehicle such as a construction machine, etc. In the device of the invention, a microprocessor (29) judges whether or not clutches H, L and clutches R, 1st, 2nd, and 4th are engaging each other in accordance with the clutch pressure instruction (C1-C7), and whether or not these clutches are selectively engaged in accordance with said clutch pressure instruction and each shaft (2a, 7a, 8a) in front and rear of these clutches is rotating in accordance with the reduction ratio corresponding to these selected clutches and, based on the results of the judgement, accurately specifies whether the fault spot of the speed change system is at some place in the electric system (sensors 9, 11, 19-25) or at some place in the mechanical system (clutch valves 12, 13, 14-18). The device of this invention is mounted on a vehicle such as a construction machine.