An electronic control system has a microcomputer monitoring section and an output controlling section. A microcomputer is prohibited from controlling control objects of a vehicle by a control prohibition section in the output controlling section and monitoring halting section halts the abnormality monitoring program executed by a monitoring section, when there is a heavy-load program processing request. Thus, the microcomputer executes the heavy-load program processing with reduced processing load. The abnormality monitoring is started by the monitoring section, when the processing of the heavy-load program is terminated. Further, after normal operation of the microcomputer is checked, the control system allows the microcomputer to start controlling the control objects.