Abstract: A cabin is movable along the outer side surfaces of an operator cab floor by a vertical drive means. Specifically, pinions fixed to left and right end portions of a shaft are driven and rotated by motors installed on the operator cab floor. Rack gears engaged with the pinions are fixed to the cabin. Even when the cabin is moved and situated at its lowermost position, where the lower edges of the cabin project downwards below the operator cab floor, by the rack gears by rotations of the pinions, a single door which is continuous in the vertical direction can be installed on a side plate. Therefore, even when the whole height of the cabin is lowered to its limit, the door can be opened and closed without troubles. The shaft also functions to synchronize the rotations of the left and right pinions mechanically, thereby preventing torsion of the cabin.