An output limiting control is carried out for limiting an output of a driving source when an acceleration pedal as well as a brake pedal is operated at the same time. An acceleration opening degree for a malfunction diagnosis is set based on a detected acceleration opening degree. A required output value for the malfunction diagnosis is calculated based on the acceleration opening degree for the malfunction diagnosis. A malfunction determining threshold is set in accordance with the required output value. An estimated output value of the driving source is compared with the malfunction determining threshold in order to determine whether there is a malfunction in a control system for the driving source. The acceleration opening degree for the malfunction diagnosis is limited to a predetermined limiting value, when the acceleration and brake pedal are operated at the same time.