A microprocessor 20a controls an electrical load group 12 responsive to content of a non-volatile program memory 25a and operation state of an input sensor group 11. A monitoring control circuit section 30a sequentially transmits a large number of question items with an inquiry packet, and compares response content from the microprocessor 20a with correct answer information to carry out an error determination. The microprocessor 20a diagnoses an interval of receiving an inquiry packet to monitor in reverse the monitoring operation of the monitoring control circuit section 30a. Thus, in an electronic control unit having a microprocessor built-in, a monitoring control circuit is obtained that alternatively executes at regular intervals a part of control programs to carry out operation inspection during operation.