A proximal portion of a first link is pivotally supported by a rear end portion of a frame. A second support shaft is provided in a distal portion of the first link. A first cylinder and a second link to which a counterweight is connected are supported by the second support shaft. A second cylinder is connected to the second link. Through pivoting of the second link by the second cylinder, the counterweight is selectively raised and lowered between a mounting position defined on the frame and a separating position above the mounting position. Through pivoting of the first link by the first cylinder, the counterweight moves between the separating position and the ground surface.