To provide a theft preventing device for a vehicle to prevent the occurrence of the theft of a vehicle even when a case to contain an engine control part is decomposed and a main computing processing device is ruptured to operate a limp-form function circuit.

A limp-form decision circuit 53 always monitors the operation state of a CPU 43 through a monitoring wire 59. In which case, when operation of the CPU 43 is brought into an abnormal state, when an input 57 from a contact switch 31 is in a 'High' state, the cover of a case to contain an engine control part 11 is opened and a decomposition state is produced. Since intentional trouble occurs, the limp-form decision circuit 53 does not switch an input from a gate circuit 55 and an output 47 from the CPU 43 is left as it is. As a result, an engine control part 11 is prevented from switching over to a limp- form function circuit 49.