To provide a backup device for an air compressor control device of a low failure rate and a high reliability at the minimum cost.

A backup circuit 21 composed only to conduct main control necessary for operation of an air compressor is connected in parallel to a CPU 20, and when the CPU has abnormality, output of the CPU is selected to the output of the backup circuit 21 by a selector means 23. Comparing with the case using a CPU for the backup circuit 21, the number of parts can be reduced, and improvement of reliability and reduction of cost can be achieved. In addition, by providing an abnormality detection means 22 separately provided from the CPU to actuate the selector means 23 by detecting abnormality in the CPU, the CPU can be securely changed into the backup circuit 21 even when the CPU 20 runs away to be incapable of accepting an interruption signal, thereby countermeasures for the failure of the CPU become complete.