When control to maintain a constant engine rotation number is executed, the electronic control unit constantly monitors an error of detected throttle position data with respect to a desired throttle control value, executes a first stage of a failure judgment method to judge that a failure of a throttle control system has occurred in a case where an absolute value of the error exceeds a predetermined threshold value continuously for a predetermined time or longer, and executes a second stage of the failure judgment method to judge that a serious failure has occurred in the system in a case where a detected engine rotation number exceeds a predetermined rotation number continuously for a predetermined time or longer, to perform predetermined operations in accordance with the judgment results.