PURPOSE: To surely detect a fault in either of digital computers and their input systems.

CONSTITUTION: A control device for internal combustion engine is provided with a pair of calculating digital computers ECC1 and ECC2, and a diagnostic digital computer DECU. In the digital computers ECU1 and ECU2, the control amount of an engine is calculated based on signals input from multiple sensors 30a to 35a and 30b to 35b, and the calculated control amount is transferred to the digital computer DECU. In the digital computer DECU, an engine speed NE is calculated based on pulse signals input from NE sensors 30a and 30b, and a predicted control amount predicted from the engine speed NE is compared with the control amount transferred from the digital computers ECU1 and ECU2. When there is a difference between them, it is judged that either of the digital computers ECU1 and ECU2 and their input systems is faulty.