An upper frame swingable between an inclined posture and a substantially horizontal posture along a trajectory of movement of a four-bar linkage is held in the substantially horizontal posture by a simple operation when necessary. A front link mechanism control gives an external force for pushing down an operation member to thereby control a front link mechanism so as to displace the upper frame rearward while lowering a front side of the upper frame. Since a displacement direction for bringing the upper frame into the substantially horizontal posture and an operation direction for locking the front link mechanism or a rear link mechanism by the operation of the operation member are substantially the same direction, it is possible to perform two movements, the displacement movements to the substantially horizontal posture and the locking movements, only by operating the operation member in a predetermined direction.
