PURPOSE: To ensure the high reliability of a computer without using any special hardware by separating a master unit when it has a fault of the synchronizing transmission and at the same time substituting one of slave units for a master unit.

CONSTITUTION: A computer contains three arithmetic control units 1aW1c that perform the same processing and a majority logical circuit 2. These units 1aW1c are connected to each other by the 1st and 2nd transmission lines 3 and 4. The external sensor signals, etc. are supplied via an input line 5, and supplied to the circuit 2 via a unit output line 6 after a prescribed operation. Then the sensor signals are outputted via a control output line 7 and also fed back via a control output monitor line 8. The units 1aW1c collates these signals with own output and feed them back. When a rise indication is given to a unit, a master unit is obtained in terms of the synchronizing control. If this master unit has a fault, a slave unit substitutes for the faulty master unit. Thus the normal working is virtually secured from outside.