To realize the safety of operation, by discriminating automatically the adequacy of the change gear shift position operated selectively by an operator by a shift control device, in the running shift operation of a tractor.

The shift position prior to the operation of a mode operation lever 77 by which a car speed suitable for the work is selected, is compared with a car speed expected after the operation, so as to discriminate whether it is an adequate operation or an error operation and when it is an adequate operation, an instruction is output to engage the shift clutches 4 to 7 by the operation, but when it is an error operation, a shift control device 80 not connecting and outputting the shift clutches 4 to 7 temporarily is provided.