To simplify installation work by easily supplying motive power to an extension conveyor when installing the extension conveyor in a self-traveling civil engineering/construction machine.

Hydraulic lines 20 and 21 of a discharge conveyor are detachably formed by interposing hydraulic couplers 23a and 23b in the side vicinity of a first hydraulic motor 16. Hydraulic couplers 39a and 39b are installed on the end of hydraulic lines 37 and 38 of a second hydraulic motor 31 for driving the extension conveyor, and are formed so as to be connectable to the hydraulic couplers 23a and 23b of the discharge conveyor. When using the extension conveyor, both hydraulic lines are connected by the hydraulic couplers 23a, 39b and 39a, 23b, and the first hydraulic motor 16 and the second hydraulic motor 31 are connected in series, and pressure oil is supplied to both hydraulic motors 16 and 31 from the same hydraulic pump 18.