In order to hold an upper frame capable of oscillating along the motion trajectory of a quadric crank chain that takes an inclined posture and an approximately horizontal posture in the approximately horizontal posture by a simple operation when necessary, an upper frame (20) capable of oscillating along the motion trajectory of a quadric crank chain between an inclined posture and an approximately horizontal posture is forcibly displaced to the approximately horizontal posture and held. A front link mechanism control means (100) applies external force for pushing down an operation member (120) to thereby control a front link mechanism (30) such that the front side of the upper frame (20) is displaced backward while being lowered. The direction of the displacement for bringing the upper frame (20) into the approximately horizontal posture and the direction of an operation for locking the front link mechanism (30) or a back link mechanism (40) by the operation of the operation member become approximately the same direction as each other, and consequently two movements, a displacement movement to the approximately horizontal posture and a lock movement, can be performed only by operating the operation member in a predetermined direction.Afin de maintenir un cadre supérieur apte à osciller le long de la trajectoire de mouvement dune chaîne de manivelle quadrique qui prend une posture inclinée et une posture approximativement horizontale dans la posture approximativement horizontale par une opération simple lorsquelle est nécessaire, un cadre supérieur (20) apte à osciller le long de la trajectoire de mouvement dune chaîne de manivelle quadrique entre une posture inclinée et une posture approximativement horizontale est déplacé de façon forcée vers la position approximativement horizontale et maintenu. Un moyen (100) de commande dun mécanisme de liaison avant applique une force externe pour pousser vers le bas un élément de fonctionnement (120) pour commander par là un
