PURPOSE: To improve safety at the time of work by prohibiting changeover between an automatic travel mode and a manual travel mode under a fixed condition.

CONSTITUTION: This device is provided with a first detecting means 14 to detect that a clutch 1 is in the state of having been cut off by means of an automatic clutch device 5 and a second detecting means 15 to detect that wheels on a turning direction inner side are in the state of having been braked by means of an automatic brake device 6, at the time of automatic travel mode selection, and in a case in which at least one of the detecting means 14, 15 is conducting detection, changeover to a manual travel mode from a automatic travel mode is prohibited by means of a travel mode changeover prohibiting means.